United States Department of Commerce

Technology Administration

National Institute of Standards and Technology

NAT'L INST. OF STAND & TECH R,LC.

NIST Special Publication 834 ~ NiST

REFERENCE PUBLICATIONS

Ceramic Macnimng: Assessment of Current Practice

and Research Needs in the United States
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NIST Special Publication 834

Ceramic Machining: Assessment of Current Practice

and Research Needs in the United States

S. Jahanmir

L. K. Ives

A. W. Ruff

M. B. Peterson

Ceramics Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

Issued June 1992

U.S. Department of Commerce

Barbara Hackman Franklin, Secretary

Technology Administration

Robert M. White, Under Secretary for Technology

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office

and Technology Washington: 1992

Special Publication 834

Natl. Inst. Stand. Technol.

Spec. Publ. 834

106 pages (June 1992)

CODEN: NSPUE2

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
EXECUTIVE SUMMARY

Advanced structural ceramics, such as silicon nitride, are attractive for many advanced

applications due to their high strength at elevated temperatures, resistance to chemical degradation, wear

resistance, and low density. Despite these advantages, there are considerable impediments to the

introduction of advanced ceramics. With current technology, fabrication costs are high, compared to

other materials, and component reliability is uncertain. The cost of machining can be as high as 90

percent of the total cost of some high precision components; and damage produced during machining can be detrimental to the performance and can produce premature failure.

A study was conducted to assess the current state-of-the-art in the machining of advanced

ceramics and to identify research areas which could lead to significant improvements. In conducting the

assessment, an extensive literature search was carried out, visits and discussions were held with industrial

companies interested in ceramic machining, a telephone survey was conducted on ceramic machining

shops, a research-in-progress database was consulted, individuals were invited to visit NIST and discuss

different aspects of ceramic machining, and a workshop was held at NIST in September 1990, to identify

specific industrial needs in ceramic machining.

The results of the assessment with regards to the needs in the machining and production of parts

made from advanced ceramics is summarized as follows:

Reduction in Cost: The cost associated with machining is a major ontributor to the overall high

cost of advanced ceramic components. Although^significant costs are also associated with raw

materials, processing, and quality control, the primary impediment to the introduction of most

advanced ceramic parts into the market place is the high cost of machining. Therefore, research

efforts should be focused on developing methods and procedures for cost-effective machining of

ceramic components.

Optimization Data: With the exception of a few publications, there is little machining data

available for advanced ceramics. There is a critical need for data to be used for the purpose of

cost estimating, component design optimization, and selection of machining parameters. In the

absence of such information, the operator takes the most conservative route, i.e., slow removal

rates and a concomitant increase in the cost of machining.

Rapid Machining Methods : Advanced ceramic materials are difficult to machine with most

conventional methods, with the exception of abrasive machining utilizing diamond. Abrasive

machining of advanced ceramics, however, is slow compared to metals. Innovative techniques

are needed to increase the rate of material removal. But it is extremely important to recognize

that an increase in machining rate may lead to an unacceptable level of surface and subsurface

damage in the finished part. Therefore, the development of rapid machining methods must be

accompanied by damage assessment and control.

Damage Assessment: Performance and reliability of ceramic components are strongly influenced

by damage introduced during machining. Surface finish and machining damage are especially

sensitive to small changes in machining conditions. There is a need for reliable and fast in situ

and post-machining damage assessment techniques. Development of sensors for real-time

iii
measurement of surface finish and damage assessment has the potential to increase manufacturing

productivity significantly.

Automation: Present ceramic machining practice is oriented towards labor-intensive small-scale

production and heavy reliance on operator skill. If advanced ceramics are to be extensively used,

automated production techniques are needed to make millions of components. This requires

additional research to develop new machine tools, special part-handling techniques, and on-line

inspection to control machining damage, surface finish and part dimensions.

In order to take advantage of the unique properties of advanced ceramics and facilitate their

introduction into the market place, a coordinated research program is needed. The overall objectives of

this program should be to develop an advanced technology for machining of advanced ceramics. Based

on our discussions with industry and the analysis of published literature, we recommend a broad research

program, consisting of the following areas:

• Optimization of the Grinding Process

• Chemically Assisted Machining

• Grinding Wheels for Machining of Ceramics

• Direct Damage Assessment Techniques

• NDE Techniques for Machining Damage Evaluation

• Sensors for Real-Time Surface Finish and Damage Assessment

• Standard Reference Materials

• Post-Machining Treatments for Damage Control

• Innovative High-Strength Machinable Ceramics

• Automated Systems for Large-Volume Production

The ultimate goal of this program is to further the utilization of advanced structural ceramics in

industrial applications by increasing the cost-effectiveness of ceramic components. It is recommended

that these projects be carried out jointly by government laboratories and industry to facilitate technology

transfer, and that all research activities funded by the government be coordinated to minimize the

possibility of duplication of effort.

iv
ACKNOWLEDGEMENTS

Many individuals representing different industrial companies and academic institutions were

contacted for information during the course of this assessment. We would like to acknowledge their

willingness to provide the requested information and thank them for their encouragement. In addition

to the individuals who were formally contacted, many others provided information in informal

conversations, which increased our awareness of the problems in ceramic machining. Particularly, the

cooperation of the United States Advanced Ceramics Association is appreciated.

We are grateful to the NIST Ceramics Division and the Precision Engineering Division Staff, for

their support and encouragement. We would like to give a special thanks to S. M. Hsu, S.J. Dapkunas,

C. Evans, and J. Carpenter.

DISCLAIMER

Information on product names, manufacturers, or suppliers are included in this report for

clarification. This does not imply endorsement of the products or services by NIST.

COVER PHOTOGRAPH

The cover photograph is courtesy of Robert K. Kopp, Norton Company, Worcester, MA.

v

TABLE OF CONTENTS

EXECUTIVE SUMMARY iii

ACKNOWLEDGEMENTS v

1. INTRODUCTION 1

2. MACHINING METHODS AND PRACTICES 7

2.1 Abrasive Methods 8

2.1.1 Grinding 8

2.1.2 Honing 10

2.1.3 Lapping and Polishing 11

2.1.4 Ultrasonic Machining 12

2.1.5 Liquid Abrasive Jet Cutting 13

2.1.6 Single-Point Turning 14

2.2 Nonabrasive Methods 15

2.2.1 Electrical Discharge Machining 15

2.2.2 Laser Beam Cutting 15

2.2.3 Electron Beam and Ion Beam Cutting 17

2.2.4 Friction Cutting and Microwave Cutting 17

2.3 Combined Methods 18

2.3.1 Electrochemical Grinding 18

2.3.2 Thermally Assisted Turning 19

2.3.3 Mechanical-Electrical Discharge 19

2.3.4 Chemical-Electrical Discharge 19

3. ASSESSMENT OF INDUSTRIAL NEEDS AND CURRENT RESEARCH 21

3.1 Current Industrial Practice 21

3.2 Ceramic Machining Research in Industry 23

3.3 Industrial Needs 24

3.4 Current Government Programs 24

3.5 Research Activities in Other Countries 25

3.6 Recent Publications 25

vii
4. OPTIMIZATION OF THE GRINDING PROCESS 27

4. 1 Fundamentals of Abrasive Removal 27

4.2 Elements of the Grinding System 29

4.2.1 Machine Tool 29

4.2.2 Grinding Wheel 30

4.2.3 Truing and Dressing 31

4.2.4 Grinding Fluid 33

4.3 Suggested Technical Approaches 34

5. CHEMICALLY ASSISTED MACHINING 37

5.1 Chemical Effects in Machining 37

5.2. Chemical Effects in Drilling 37

5.3 Chemical Effects in Polishing 43

5.4 Chemically Assisted Grinding 47

5.5 Chemically Assisted Liquid Abrasive Jet Cutting 49

5.6 Chemically Assisted Ultrasonic Machining 49

5.7 Chemically Assisted Single-Point Diamond Turning 49

5.8 Suggested Technical Approaches 50

6. MACHINING DAMAGE AND DETECTION 51

6.1 Mechanisms of Damage Formation in Grinding 51

6.2 Grinding Rate and Damage 53

6.3 Methods of Damage Characterization 58

6.4 Suggested Technical Approaches 63

7. RECOMMENDED RESEARCH FOR CERAMIC MACHINING 65

7.1 Summary of Needs 65

7.2 Recommended Research Projects 66

7.2.1 Optimization of the Grinding Process 66

viii
7.2.2 Chemically Assisted Machining 66

7.2.3 Grinding Wheels for Machining of Ceramics 67

7.2.4 Direct Damage Assessment Techniques 67

7.2.5 NDE Techniques for Damage Evaluation 68

7.2.6 Sensors for Real-Time Surface Finish and Damage Assessment 68

7.2.7 Standard Reference Materials 68

7.2.8 Post-Machining Treatments for Damage Control 69

7.2.9 Innovative High-Strength Machinable Ceramics 69

7.2.10 Automated Systems for Large-Volume Production 69

7.3 Program Coordination 70

7.4 Technology Transfer 70

8. REFERENCES 71

Appendix A NIST CERAMIC MACHINING WORKSHOP 85

Appendix B ATTENDANCE AT THE NIST CERAMIC MACHINING WORKSHOP 89

Appendix C ORGANIZATIONS AND INDIVIDUALS CONTACTED FOR INFORMATION 93

Appendix D ADDITIONAL BIBLIOGRAPHY 95

ix

1. INTRODUCTION

Advanced structural ceramics, such as silicon nitride, are attractive for many advanced

applications due to their high strength at elevated temperatures, resistance to chemical degradation, wear

resistance, and low density. These materials are currently being used, or proposed for, a wide variety

of applications; some are listed in Table 1. Electronics, telecommunications, optical systems, sensors,

catalysts, bone replacements, heat exchangers, heat engines, and material processing equipments are all

either benefiting from, or projected to benefit from, advanced ceramic materials [Katz 1989].

The advanced ceramics industry in the United States consists of approximately 100 different

companies with sales equal to about $14 billion per year and growing at a rate of about 8 percent per year

[Eckert and Weatherall 1990]. The contribution of advanced ceramics to the ceramic industry is approximately 14. 1 percent, based on sales, as shown in Table 2. About 70 percent of advanced ceramics

are used for electronic applications, 18 percent for structural applications and 12 percent for coatings.

The use of structural ceramics is expected to grow at a rate of 4 percent per year, unless there is significant penetration into the automobile market. Then, it is estimated that the market for structural

ceramics would double every 5 to 7 years, and increase to $5 billion by the year 2000 [Office of

Technology Assessment 1988]. However, this estimate carries the caveat that the following challenges

be addressed:

• The technical feasibility of replacing metal engine parts with ceramics must be

demonstrated;

• Ceramic material and component reliability must be improved so that more

accurate design predictions can be made;

• Improved nondestructive evaluation techniques must be developed; and

• Fabrication costs must be reduced.

Considerable progress has been made in meeting the first challenge. A variety of engine wear

parts (valves, liners, cams, rings, and bearings) have been fabricated from ceramic materials and,

successfully implemented in test engines. Advances are also being made in improving the reliability of

advanced ceramics. In addition, within the past 20 years a large effort has been devoted to the

development and evaluation of nondestructive evaluation (NDE) techniques for ceramics. The basic

problem has been that quality control of ceramics requires surface flaw detection in the 10 /xm range.

This is an order of magnitude below that which is commonly used for metals. Accordingly, a variety

of new approaches have been developed. Although none of these are routinely used at present, there

appear to be no serious barriers to their use.

Undoubtedly, one of the main barriers to the use of advanced ceramics as mechanical components

is the high cost of machining when compared with currently used metal alloys. This conclusion was

confirmed at the Ceramic Maching Workshop held at the National Institute of Standards and Technology

in September 1990. See Appendix A.

The objective of the workshop was to identify the potential for cooperative research between

NIST and the ceramics industry. Representatives from the structural and electronic ceramics industry,

1
Table 1. CURRENT AND FUTURE MARKETS FOR ADVANCED CERAMICS

[Wortendyke 1989]

• Aerospace

- advanced electronics

- bearings

- combustors

- fuel cells

- fuel systems and valves

- high temperature auxiliary

power units, low weight

components for rotary

equipment such as starters

- seals

- structures

- thermal protection systems

- turbine engine components

• Automotive

- advanced reciprocating

engines

- advanced rotary regenerators

- catalytic converters

- drivetrain components

- electronic substrates

- fixed boundary

recuperators

- fuel injector

components

- low heat rejection diesels

- turbines

- turbocharger rotors

- valves and valve seats

- waterpump seals

• Bioceramics

- artificial teeth, bones

and joints

- heart valves

• Chemical Process Industry

- catalysts and igniters

- mechanical seals

- nozzles

- radiant tubes and burners

- recuperators

- reformers

- refractories

- valve components

• Defense

- armor

- bearings

- engine combustor sections

- gun barrel liners

- improved armor

- optics

- rocket nozzles

- SDI (optical/heat transfer properties)

- submarine shaft seals

• tank power trains

• Electric Power Generation

- bearings

- ceramic gas turbines

- cogeneration

- filters (gas clean-up)

- fuel cells (solid oxide)

- high temperature

components

- reactor pump seals

• Electronics

- advanced multilayer

integrated packages'

- electro-optic packaging

- multilayer capacitors

- optical wave guides

- pressure and gas sensors

- semiconductor packages

- substrates

- superconductors

• Environmental

- advanced components and

systems for environmentally

harsh processes

- filters and scrubbers

- incinerator liners and after- burners

- radiant burners and boilers

- wastewater treatment

• Metals Processing

- boats

- burners

- crucibles and ladles

- cutting tools and dies

- insulation

- molten metal filters

- seeded gel abrasives for metal/ceramic finishing

• Oil Industry

- bearings

- blast sleeves

- flow control valves

- pumps

- refinery heaters

as well as users and machine tool manufacturers were invited to this meeting and were given the

opportunity to present their views and perspectives on the need for research in ceramic machining.

Twenty-one individuals from industry participated. The United States Advanced Ceramic Association

(USACA) was also represented. Both NIST personnel and industry attendees presented research results

pertinent to ceramic machining. The following list of research needs were identified by the industrial

speakers present:

• Reduction in Cost

• Novel Processes for Machining

• Production Volume to Price Interaction

2
Mechanisms of Machining of Ceramic Parts

Effect of Machining Parameters on Performance

Creative Automation Technology

Identification and Quantification of Machine Tool Characteristics

Grinding Wheel Design

Grinding Fluid Behavior

Measurement of Surface Roughness and Quality

Grinding Machine to Surface Interactions

Accuracy of Machining Cost Projections

Table 2. SALES OF CERAMICS BY INDUSTRY

[Eckert and Weatherall 1990]

Glass 56.5%

Porcelain 14.7%

Advanced Ceramics 14. 1 %

White Ware 9.3%

Refractories 5.4%

The primary concern of the industrial participant was the high costs associated with machining

of advanced ceramics. In this regard, it is important to compare machining costs for metals and advanced

ceramics by considering the fabrication of a particular component. To carry out the comparison it must

be assumed that both materials require the same machining method, namely grinding. Clearly, if grinding

was necessary for the ceramic component, while high-speed turning or a high-rate forming process could

be utilized for the metal component, the metal would be favored. Obviously, in a real situation all costs

in producing a component would be balanced against performance requirements before making the actual

choice of material.

To demonstrate the importance and source of high machining cost of ceramics, a comparison was

obtained from Therm Advanced Ceramics, which involved the grinding of the root section of a turbine

blade. The results of their comparison providing a breakdown of the various costs are presented in

Tables 3* and 4. The advanced ceramic was silicon nitride and the metal a superalloy. The hypothetical

job involves the production of 2000 turbine blades per month over a 5-year period. The estimated cost

of grinding the silicon nitride blade root is approximately 30 percent higher than grinding the superalloy

blade root. The major contribution, amounting to 90 percent of the increase, resides in the increased

labor for grinding the ceramic parts and for dressing the grinding wheels. The increased labor for

grinding is associated with the slower material removal rates and more stringent surface finish

Tables and figures citing non-SI units are quotations from non-NIST sources.

3
Table 3. COST COMPARISON IN MACHINING CERAMIC AND METAL

TURBINE BLADE ROOT (Therm Advanced Ceramics)

Parameters

Part Tolerances 0.0004 0.0004

Surface Finish

Required

16 micro inch 32 micro inch

Operations per Part 12 12

Grinding Wheel

Geometries/Part

5 5

I,** Ul i Y\{\ inn W/Vi ool c iiiuing W IlCClo

Used

1/ liw ralia

(Diamond)

1l/^W /Oftfl Parte i alls

(Alumina)

Grinding Wheel

Dressing

0.6 h/40 Parts 0.0833 h/20 Parts

Part Setup 240 h/Lot 240 h/Lot

Lot Size 2000 Parts 2000 Parts

Notes:

1. The hypothetical job estimated herein is based on:

a. Final machining of near form cast 3" gas turbine rotor blade root stocks.

b. Therm Advanced Ceramics experience in machining such ceramic and

metal gas turbine components.

2. Cost estimates are for a 5-year run at 2000 blades/month - 120000 parts.

3. Part and production process qualification complete prior to production run.

4. Grinding is the only machining operation used.

5. Machining labor priced at $45/hour; engineering labor at $60/hour.

requirements to maintain strength of the ceramic parts. The increased labor for wheel dressing is connected with the use of diamond wheels rather than easily dressed aluminum oxide wheels which can be employed for the superalloy parts. The cost of consumables (grinding wheels and fluid) is three times

higher for the ceramic blade but amounts to only a fraction of 1 percent of the per blade cost. These

results demonstrate the importance of developing rapid machining capabilities for ceramic materials if ceramics are to compete directly with metals.

Therm Advanced Ceramics also prepared a cost estimate for smaller production runs, Figure 1. When the run is limited to only 10 to 100 parts, the cost per part is extremely high for both the ceramic

and the metal components. This is a result of the large contribution made by the setup and nonrecurring

4
Table 4. COST COMPARISON IN MACHINING CERAMIC AND METAL

TURBINE BLADE ROOT (Therm Advanced Ceramics)

Ceramic Part Nickel Alloy Part

Time (h) Cost Time (h) Cost

Part CAD/CAM 100 $6,000 100 $6,000

Fabricate Tool and

Fixture

550 $24,750 550 $24,750

Total Setup and

Nonrecurring

Engineering

650 $30,750 650 $30,750

Pro-rata Setup and

Nnnrppurrincr

Engineering

0.005 $0.26 0.005 $0.26

Labor: Part Setup 0.120 $5.40 0.120 $5.40

Labor: Wheel Dress 0.075 $3.38 0.208 $0.94

Labor: Grinding 2.405 $108.23 1.859 $83.66

Total Consumables $0.68 $0.22

Per Blade 2.605 $117.95 2.005 $90.48

Notes: Consumables include grinding wheels and cutting fluid.

engineering expenses. However, for simpler designs, and also for metals with a greater machinability

than superalloys, parts can be made by turning or milling while an equivalent ceramic part would be

ground. Under such circumstances the ceramic part would be much more expensive.

Thus, in the absence of major technical barriers, it is clear that high machining costs impose a

significant impediment to the widespread use of advanced structural ceramics in mechanical components.

The NIST Workshop, referred to earlier, indicated that a focussed effort would be necessary to reduce

machining costs. A similar conclusion has been made in a report issued by the Oak Ridge National

Laboratory [Stinton 1988] and a workshop sponsored by the National Science Foundation [Kennedy and

Skaar 1989]. Therefore, it was decided to conduct a comprehensive assessment of the present status of

ceramic machining and subsequently to define a research program that would result in significant

improvements in machining practices and a concomitant reduction in the cost of machining.

In conducting the assessment, a comprehensive literature search was carried out, visits and

discussions were held with industrial companies interested in ceramic machining, a telephone survey was

conducted of ceramic machining shops, a research-in-progress database was consulted, and individuals

5
2000

0 I—i 1 i ' i i i

1 10 100 1000 10000 100000 1000000

NUMBER OF PARTS MACHINED

CERAMIC PART NICKEL ALLOY PART

Figure 1. Cost per part as a function of number of parts machined (Therm Advanced Ceramics).

were invited to visit NIST and discuss different aspects of ceramic machining. The assessment was

focused on obtaining answers to several questions: What is the precise nature of the problem? Is the

available data and information reliable? Are the present approaches too conservative, or are the economic

incentives for improvements insufficient? Is the current technology adequate, or is additional work

necessary? Should the current machining methods be optimized, or should new techniques be developed?

What types of research projects are needed to address the short-term and long-range machining problems?

To what extent are industry and government researchers addressing the problem? The results of the

assessment are presented in this report and provide the basis for the recommended program of research.

6
2. MACHINING METHODS AND PRACTICES

Forming methods that require extensive plasticity such as stamping, forging and drawing, which

are of great importance in metal component fabrication, can only be applied to advanced ceramics in the

green state. In principle, given an appropriate atmosphere, high pressure, and elevated temperature,

plastic working of advanced ceramics could be accomplished. Under ordinary conditions, however,

advanced ceramics exhibit limited ductility even at temperatures close to the melting point. Research in

progress [Hahn et al. 1991] has shown that certain nanocrystall ine ceramics can be worked at temperatures as low as 600°C - 800°C as a result of grain boundary diffusion-controlled creep processes.

Strain rates as high as 10"4 s"

1 can be achieved. This is high for creep processes in ceramics, but is still extremely low compared to plastic forming rates of metals. Therefore, machining is the only viable

fabrication method applicable to advanced ceramics in the fully sintered state.

A wide variety of methods that are currently being used, or have been proposed for machining

of advanced ceramic components, are reviewed in this chapter. These methods, which are classified as abrasive, nonabrasive. and combined processes , are listed in Table 5. They are not unique to advanced

ceramics and indeed were developed primarily for application to metals. Some of these methods such

as grinding, honing, lapping, polishing, and ultrasonic machining are extensively used for ceramics.

Others have been proposed and require additional research and development. Turning and milling

methods, although extensively used in metal machining, have only limited application to advanced

ceramics in their fully sintered state because of high rates of tool wear and severe workpiece surface

damage. However, turning and milling principally with polycrystalline diamond tools are employed quite

effectively to machine advanced ceramics in the green state. Unfortunately, this does not eliminate the

need for finish machining of densified ceramic parts.

Table 5. MACHINING METHODS FOR ADVANCED CERAMICS

ABRASIVE METHODS NONABRASIVE METHODS

Grinding

Honing

Electrical Discharge Machining

Laser Beam Cutting

Electron Beam and Ion Beam Cutting

Friction Cutting and Microwave Cutting

Lapping and Polishing

Ultrasonic Machining

Liquid Abrasive Jet Cutting

Single-Point Turning

COMBINED METHODS

Electrochemical Grinding

Thermally Assisted Turning

Mechanical-Electrical Discharge

Chemical-Electrical Discharge

7
2.1 Abrasive Methods

2.1.1 Grinding

Many different modes of grinding can be identified, i.e., reciprocating, internal-cylindrical,

external-cylindrical, centerless, creep-feed, etc. However, they all have as a common element the use of a circular wheel or tool that is rotated about its axis of symmetry with some part of the periphery

sliding against the workpiece. The entire tool, a layer a few millimeters thick, or only a single layer such

as in plated tools, consists of bonded abrasive grains. The tool may range in size from a tiny

submillimeter abrasive coated drill to massive grinding wheels more than a meter in diameter. A variety

of different abrasives are used. For metals the most common are aluminum oxide and silicon carbide.

For application to advanced ceramics only the superabrasives, diamond and cubic boron nitride (CBN)

are used, with diamond being the primary choice. Cubic boron nitride, although significantly harder than

other advanced ceramics, has been shown to wear much more rapidly than diamond [McEachron and

Lorence 1988].

Because of the complexity of grinding, it is common to view the process in terms of several

separate but interdependent elements. Metzger [1986] in his book on superabrasive grinding identifies

three elements: the workpiece, the machine, and the grinding wheel. Recognizing the importance of the

grinding fluid and the dressing tool, Cooley and Wapler [1984] list a total of five elements, shown in

Figure 2. In a production setting all of the factors identified in Figure 2 must be considered and

optimized interactively to maximize cost-effectiveness. This procedure is quite familiar to the machining

industry where it is applied in the grinding of metals, cermets, and a variety of other conventional

materials. Significant advances within the various elements will be required, however, before a comparable level of capability is reached with advanced ceramics.

Input Data

SYSTEM

Machine - type - properties

Workpiece — geometry — material

Grinding Wheel - geometry — composition

Dressing tool - type

Coolant - type - application

PARAMETERS

infeed

table feed

workpiece

speed

cutting

speed

dressing

conditions

pressure

flow rate

Grinding

Process

PROCESS DATA

force

power

wear

temperature

process time

I

Disturbance data — vibration - temperature

Result

TECHNOLOGY

Workpiece — shape accuracy — dimension — surface finish — micro depth — disturbance

Grinding wheel — wear — loading

Coolant — contamination — change

| Economy

Throughput

Production costs

Figure 2. Grinding system relationships [Cooley and Wapler 1984].

8
One mode of grinding which is particularly promising for application to advanced ceramics is creep-feed grinding. With this method an increased material removal rate may be achieved without

sacrificing strength and surface finish [Kong et al. 1991]. In creep-feed grinding, a very large depth of

cut is taken at low advance speeds. With brittle materials, down-grinding is employed to maintain much

of the material in compression as illustrated in Figure 3. As the workpiece moves into the wheel, the

bulk of the material is removed at the front of the wheel. The forces are also highest at that location.

Progressing towards the bottom of the wheel, the forces fall off rapidly and the effective grinding depth

is reduced. In this way damage is minimized and surface roughness is reduced. This was clearly

demonstrated by Konig and Wemhoner [1989] in grinding experiments on siliconized silicon carbide.

The effect of depth of cut and feed rate on damage are illustrated in Figure 4. In this example, material

removal rate was maintained constant by balancing feed rate and cutting depth. Characteristic strength

is significantly higher at large depths of cut and low feed rates.

1 3

Figure 3. Schematic of creep-feed grinding [Subramanian 1987].

Application of creep-feed grinding is not achieved without complications. Since the grinding

forces are considerably higher, a more powerful machine with a greater machine rigidity is required to

avoid excessive deflections and associated loss of accuracy. The workpiece itself must, of course, be able

to support the increased loads, and greater care is required in fixturing. The long wheel contact length

and high specific grinding energies also place increased demands on the grinding fluid, and its delivery.

To address the delivery problem, special slotted wheels have been designed to increase the access of the

grinding fluid and have been shown to be quite effective [Suto et al. 1989]. However, no attempts have

been made to develop special fluids for creep-feed grinding of ceramics. It is clear that applying creepfeed grinding technology to advanced ceramics offers a significant opportunity to obtain increased

machining rates without sacrificing strength or component integrity.

9
270

MPa

m = 12.2 10.8

b°

XI

-*->

tSfi ti

CDu->-> m

o•r-i

-t-> W• i-f

f-, <D +JO

(0

cd xlo

240

210

m = 11.2

15.0

180

oIX i_I II T T XT

V

ft

0.03

167

0.1

50

0.25

20

0.5 mm

10 mm/s

Figure 4. Strength of siliconized silicon carbide at constant removal rate as a function of infeed (a,,) and

feed rate (vft), adapted from Konig and Wemhoner [1989].

2.1.2 Honing

Honing, like grinding, also uses fixed abrasives; and for application to advanced ceramics,

diamond is the abrasive of choice. Much of the discussion on grinding also pertains to honing. The main

difference between honing and grinding is the much lower surface speeds employed in honing. Honing,

in general, is not used to remove large amounts of material. Its primary application is in obtaining a desired surface finish and in correcting errors in dimensional tolerance left by other more rapid machining

methods. The most frequent application of honing is in the finishing of internal cylindrical surfaces; the

honing of automobile engine cylinder walls being a most notable example. Applications such as the latter

may be highly automated with complete control over all aspects of the process, including automatic

gaging of the part dimensions. Honing is also applied to external surfaces, for example, in finishing gear

teeth, valves, and the outside surfaces of ball and roller bearing races.

A review of honing methods and applications can be found in the Metals Handbook [Davis 1989].

Only brief mention is made of ceramics. As with grinding, the honing of ceramics requires diamond

abrasives, and there is a similar concern about workpiece damage.

One advantage honing has over grinding is in the small amount of heat produced, since honing

is a low-speed operation compared to grinding. Thus, damage and workpiece distortion associated with

heating are minimized. The fact that honing involves little heating also influences the cutting fluid

10
requirements. There is less concern about cooling requirements; but additives are needed to impart

lubricity. As in grinding, it does not appear that there have been special efforts to develop honing fluids

specifically for application to advanced ceramics.

2.1.3 Lapping and Polishing

Similar to honing, lapping is largely a finishing process employed on parts that have already been

machined to near final dimensions. Lapping differs from honing in that it is a loose or free abrasive

process. Lapping is conducted by pressing the workpiece against a rigid surface, often cast iron, covered

with a slurry of abrasive particles. Some of these particles are embedded into the lapping tool surface

and produce a cutting action on the workpiece. Other particles that roll between the two surfaces, are

also involved in the removal process. A distinction that is sometimes, but not always, made between

lapping and polishing is in the use of a soft and flexible surface for polishing and a harder and rigid

surface for lapping. Lapping is employed to obtain precise tolerances, remove damage, and improve

surface finish. Polishing is used primarily to remove damage and achieve very smooth surfaces. Figure

5 shows a schematic of these processes.

(a)

(b)

Figure 5. Sketch of abrasive grit held in (a) a soft pad, and (b) a hard pad [Rostoker et al. 1987].

Recent developments in lapping and polishing of advanced ceramics have focussed on improved

sizing of abrasive grit, on combined chemical and mechanical processes, and on machine design

improvements [Rostoker et al. 1987, Subramanian 1988, and Konig and Popp 1988]. The importance

11
of grit size uniformity is clear. With a narrow size distribution, the chance of scratching the polished

surface with an oversized abrasive particle is minimized. Grit size uniformity is also crucial in order to

achieve high precision and flatness in lapping and polishing operations.

Machine design changes in recent years have emphasized close tolerance lapping and polishing,

and high specimen throughput. Modern high precision lapping machines emphasize a firm substrate for

the abrasive so that close tolerances can be maintained, and this also necessitates uniform abrasive size.

Automated operation is emphasized, and a reduction in the number of steps is sought in the usual

sequence of abrasive changes from coarse to fine size to improve production efficiency [Rostoker et al. 1987]. Stowers et al. [1988] have reviewed some recent developments in polishing, which included float

polishing, elastic emission machining and magnetic fluid polishing. Although this review is directed

toward finishing processes for large optical components, some of the techniques may be applicable to

advanced ceramics.

In float polishing, the workpiece floats on the polishing fluid as a result of hydrodynamic action

at high rotational speeds [Namba and Tsuwa 1977, 1978, 1980a 1980b, 1980c, and Bennett et al. 1987].

Elastic emission machining is based on a the same principle as the float polishing [Mori et al. 1983, Mori

and Ikawa 1985, Mori and Yamauchi 1987, and Mori et al. 1987]. The primary difference between this

method and float polishing is that a rotating polyurethane sphere is used for the lap, as compared to a hard tin disc used in float polishing.

Two variations of magnetic fluid polishing have been developed. Saito and Niikura [1987] have

used a magnetic fluid (containing abrasive particles) to form a polishing pad in which the magnetic fluid

levitates and forces the abrasive grains against the workpiece surface. In the process developed by

Kurobe and Imanaka [1984], the polishing compound is pressed against the workpiece by a thin rubber

sheet separating the polishing compound from the magnetic fluid. The magnetic fluid polishing process

has been applied to polishing of curved surfaces of lithium niobate [Suzuki et al. 1989] and silicon nitride

balls [Umehara and Kato 1990]. Although lapping and polishing are important in ceramic machining

processes and the technology is well advanced, there appears to be some potential for improvements by

chemical assistance, which is discussed later.

2.1.4 Ultrasonic Machining

Ultrasonic machining encompasses rotary ultrasonic machining and ultrasonic impact machining.

In both methods the tool is vibrated along its axis normal to the surface of the workpiece at a frequency

of typically 20 kHz. Otherwise the two methods differ substantially. Rotary ultrasonic machining utilizes

diamond coated grinding tools, most often core drills, in a drilling or vertical milling configuration. It

is distinguished from ordinary milling and drilling operations only in that the rotating tool is vibrated with an amplitude of 0.025 to 0.05 mm while being held against the workpiece. The vibration serves to reduce friction, assist in the access of cutting fluid, and facilitate the removal of swarf. The net result

is an increase in machining rate. The tools are generally limited to a diameter no greater than 50 mm.

In addition to core and solid drills, rotary ultrasonic machining has also employed small grinding and

thread cutting wheels. Clearly much of what was discussed in grinding is also applicable to this method.

The influence of grinding parameters, bond and abrasive properties, and cutting fluids should bear a similar importance. Thus, improvements in the grinding process for ceramics should also be applicable

to rotary ultrasonic machining.

12
In ultrasonic impact machining the tool itself does not incorporate abrasive particles and does not

contact the workpiece. In this method an abrasive containing slurry is circulated between the vibrating

tool and the workpiece, Figure 6. The vibratory motion of the tool is imparted to the fluid and abrasive

particles. Impact of the particles with the workpiece results in indentation and fracture with the

consequent removal of material. The rate of removal is extremely sensitive to the gap between the

workpiece and the vibrating tool. Next to grinding, ultrasonic impact grinding is probably the most

frequently used machining method for advanced ceramics. Recent progress using this approach has been

reviewed by Rhoades [1985]. More rapid machining may be possible with special fluids and higher

vibration frequencies.

Holder

Workpiece Slurry

Tool

I

+ abrasive (SIC)

Vibration amplitude

at 0.025 mm

(0.001 inch)/

frequency at

20 #000 cycles/second

Figure 6. llustration of the ultrasonic impact machining, adapted from Rhoades [1985].

2.1.5 Liquid Abrasive Jet Cutting

Jordan [1986] has reviewed recent developments and applications of liquid jet cutting. Liquid

jet systems are usually applied to cutting tasks, as illustrated in the schematic sketch in Figure 7, rather

than to shaping or surface finishing. This method has been frequently applied to porous materials where

very high cutting rates have been reported [Kim et al. 1987]. However, liquid jet cutting is not effective

on hard and dense advanced ceramics, where cutting rates are much lower.

13
Accumulator

Controls

Drain

Figure 7. Schematic of liquid jet cutting apparatus, after Firestone [1979].

Addition of abrasive grit to the fluid stream has enabled the application of liquid jet cutting to advanced ceramics. This approach combines slurry erosion removal processes with localized fracture due

to liquid cavitation. The incorporation of abrasive particles, however, will introduce an additional

element of machining damage appropriate to erosion. The liquid abrasive jet method does not easily lend

itself to the fabrication of components requiring a close tolerance and a fine surface finish.

2.1.6 Single-Point Turning

Certain soft ceramics, porous ceramics and ceramics in a sub-sintered or green state may be

machined by single-point turning and milling methods. In these operations, wear and fracture of the tool are major concerns. A dull or damaged tool results in degradation of tolerance and inflicts considerably

more damage to the workpiece. To minimize tool wear, sintered polycrystalline diamond tools are often

employed.

During the last decade, considerable attention has been focused on single-point diamond turning

process, which utilizes a polished single crystal diamond tool and requires an especially designed machine

tool to allow very close control of the depth of cut and machine tool vibrations [Schinker 1991, Sugita

et al. 1983, Veda et al. 1983, Veda et al. 1991, and Nakajima et al. 1989]. Single-point diamond turning

14
was developed as a technique for producing highly smooth and precise surface contours for specialized

applications, for example optical components and contact lens molds. Since the process is inherently

adaptable to computer control for producing curved surfaces in an automated manufacturing environment,

it will likely see expanded use in the future.

The machining geometry is shown schematically in Figure 8. In general, elastic-plastic

deformation and microfracture (especially in brittle materials) occur in the workpiece ahead of the tool

edge. If the conditions are established properly, most of the microfracture damage that occurs at a depth

up to teff is subsequently removed as part of the chip. Single-point diamond turning has been successfully

applied to a limited number of materials in producing very close tolerance surfaces of high smoothness

and low levels of damage [Bifano et al. 1988, and Blake and Scattergood 1990]. Other materials that

oxidize or dissolve diamond (carbon), such as steels, cannot be cut in this way, unless special precautions

are made to reduce the wear of the diamond tool [Evans 1991]. Application of single-point diamond

turning to advanced ceramics also requires special precautions to minimize tool wear.

2.2 Nonabrasive Methods

2.2.1 Electrical Discharge Machining

Many applications of electrical discharge machining (EDM) to advanced ceramics have been

reported in the past 5 years. EDM requires the workpieces to have an electrical resistivity less than

100Q-cm. Thus, EDM can not be used for machining of glasses and some ceramics. Koenig and Panten

[1988] have used EDM on silicon-infiltrated silicon carbide. Iwanek et al. [1986] have applied this

method successfully to siliconized SiC and hot-pressed SiC materials. One means, to extend the

application of EDM to other ceramics, has been to modify the composition of the material to reduce

resistivity. For example, the addition of TiC or TiN to silicon nitride and to alumina have produced

sufficient conductivity for EDM according to Martin et al. [1989]. Petrofes and Gadalla [1988] have also

described the application of EDM to composite ceramics, e.g., Si3 N4-TiN, SiC-TiB, and cermet materials.

Much effort appears to be underway in Japan exploring EDM for advanced ceramics and composites.

It is not clear, however, whether EDM can be regarded as a low damage machining method since it typically leaves a surface layer of melted or heat-affected material containing a high level of residual

stress and numerous microcracks.

2.2.2 Laser Beam Cutting

Focused laser beams have been used to cut a large variety of materials, including ceramics.

Chryssolouris and Bredt [1978] have reported an innovative approach that uses two laser beams

intersecting within the workpiece to cut "blind" kerfs, Figure 9. Certain cutting geometries may lend

themselves well to such a technique, particularly when segments of workpiece material need to be

removed. Copley [1985] and Wallace and Copley [1989] have described techniques for laser shaping of

ceramic surfaces such as silicon nitride.

Soni and Rajan [1985] and Kerth [1991] have described the use of pulsed lasers to drill advanced

ceramics with success. Recently, Taya and Ramulu [1988] have reported laser machining of threedimensional forms on ceramic-matrix composites.

In a study of the effect of machining on mechanical properties, Gardner and Beauchamp [1987]

reported no loss of fracture strength from laser machined alumina. Since there is no mechanical contact,

15
(per workpiece rev)

(a)

damage zones

(b)

Figure 8. (a) Schematic of cutting geometry in single-point diamond turning, (b) details of damage

formed during cutting in schematic cross section [Blake and Scattergood 1990].

16
Figure 9. Schematic illustration of a laser turning system [Chryssolouris and Bredt 1987].

the machining damage may be low, unless the material is particularly sensitive to thermal shock; in which

case, the machining damage can be extensive.

2.2.3 Electron Beam and Ion Beam Cutting

Recently reported work has involved cutting of green or low-density ceramic materials. Horio

et al. [1987] described efforts to improve the electron beam effectiveness in a pulsed system used to cut

alumina sheets. Mochel et al. [1984] described a technique to machine holes and slots in a thin alumina

sheet which they believed was suitable for nanolithography applications.

Ion beam applications have been reported by Daudin et al. [1988] in connection with surface

smoothing of alumina films by sputtering, and by Miyamoto et al. [1984] on the use of an argon ion

beam to machine alumina surfaces with possible application to biological implants. Applications of both

ion beams and electron beams in machining are only suitable for cutting and drilling thin sheets of

ceramics and, therefore, are not suitable for shaping large ceramic components.

2.2.4 Friction Cutting and Microwave Cutting

Friction cutting of ceramics has been described by Childs and Clemens [1987], where a water

cooled rotating mild steel disk was used to cut narrow slots in alumina and silicon nitride. Localized

17
microwave heating has been used by Taniguchi et al. [1972] to pierce sintered alumina. In this process,

a wafer of sintered alumina was pierced by local melting and explosive discharge of the molten material

from beneath the surface. While these methods are suitable for cutting and slicing operations, they are not generally suitable for achieving a contoured surface with a high tolerance.

2.3 Combined Methods

2.3.1 Electrochemical Grinding

Rhoades [1985] has described the use of electrochemical dissolution in combination with

mechanical grinding for machining metals. As diagrammed in Figure 10, an electrolyte (water with

dissolved sodium chloride, for example) is sprayed onto the grinding wheel and the surface being

machined. A potential is applied between a metal-bond grinding wheel (cathode) and the workpiece

(anode). Electrochemical action forms a reacted surface layer on the workpiece, which is removed by

the grinding wheel. An advantage of this method lies in the expected low level of residual damage in

the workpiece. Some advanced ceramics, notably the carbides, have sufficient electrical conductivity to permit electrochemical attack. There is no indication, however, that this approach has been tried for

advanced ceramics.

Electrolyte sprayed onto wheel

Figure 10. Sketch of the electrochemical grinding process, after Rhoades [1985].

18
2.3.2 Thermally Assisted Turning

Plasma torch heating of the workpiece during turning has been reported to be effective for some

engineering ceramics [Kitagawa and Maekawa 1990]. This method involves heating the workpiece

material to temperatures as high as 1000°C in front of a polycrystalline diamond compact (PDC) or CBN

cutting tool. Where machinability is improved, it results from a transition to a more plastic type of

deformation and removal processes at the elevated temperature. Although tool wear was reduced by a

factor of 8 in turning silicon nitride, it was still unacceptably high. Materials sensitive to thermal shock,

for example alumina and zirconia, did not show improved machinability by plasma torch heating.

In another study, successful hot-machining of mullite and silicon nitride was reported by Uehara

and Takeshita [1986]. Konig et al. [1991] have reported a method for laser-assisted turning of hotpressed silicon nitride with CBN cutting tools. Rice and Steinkuller [1975] have described the use of a

laser heating source in advance of a diamond cutting tool to render the ceramic material more plastic in

mechanical response. Nevertheless, thermally assisted turning does not lend itself to large-volume

production, because of short tool life and poor surface finish.

2.3.3 Mechanical-Electrical Discharge

The combination of electrical discharge and ultrasonic machining methods has been reported by

Uematsu et al. [1988] in experiments to machine titanium diboride with a metal bonded diamond tool.

They reported increased material removal rates and a high removal ratio of 110 (workpiece loss/wheel

loss) under the conditions chosen. Applicability of this method to other materials of sufficient

conductivity for EDM needs to be explored.

2.3.4 Chemical-Electrical Discharge

A combination of wire electrical discharge machining with electrochemical reaction in an

electrolyte has been reported by Tsuchiya et al. [1984]. This method was applied to silicon carbide and

non-conducting specimens of glass, alumina, and silicon nitride that were immersed in a suitable

electrolyte solution to permit conduction. The method permits cutting of surface contours in an effective

manner without a direct contact between the tool and the workpiece. Cutting rates for the ceramics

reportedly ranged from 0.12 to 0.14 mm/min. Further research is needed to evaluate the feasibility of

this process for large-volume production of advanced ceramics.

19

3. ASSESSMENT OF INDUSTRIAL NEEDS AND CURRENT RESEARCH

Successful implementation of advanced ceramics in industrial applications has a large implication

for broad sectors of industry. Companies most directly concerned with machining and utilization of

advanced ceramics consist of ceramic materials suppliers, component manufacturers, original equipment

manufacturers (OEM's), machine tool manufacturers, abrasives suppliers, grinding wheel manufacturers,

and the ceramic machining industry. Each industry segment has a somewhat different interest and

motivation for using advanced ceramics. Selected companies in each sector were surveyed to obtain

information on their needs and current research and development activities in ceramic machining. A

survey of government programs and published literature was also conducted to assess the current status

of research being conducted in ceramic machining.

3.1 Current Industrial Practice

In order to determine the present status of machining practice, and to assess the needs of the

ceramic machining industry, a telephone survey was conducted and several visits were made to firms in

the ceramic machining industry. The questions asked are listed in Table 6.

There are approximately 30 independent shops that specialize in the machining of ceramics. Most

of the shops are small with less than 10 employees, but a few have as many as 50 employees. In addition

to these employees, several material suppliers and manufacturers of abrasives, grinding wheels, and

machine tools have set up machine shops for ceramics, recognizing the need for improved ceramic

machining technology. These shops are well equipped and have two functions: advancing the technology

and assisting component manufacturers in the development of methods to fabricate specific components.

Almost all of the machine shops involved in ceramic machining have acquired the necessary skills for

machining of ceramics and difficult-to-machine materials. The expertise, however, is associated with

particular machinists who have acquired the skills through experience.

In addition to the general machine shops, there are a number of companies with specialized

equipment such as laser cutting, waterjet cutting, electrical discharge machining, and chemical etching,

which can be used for ceramics but are primarily used for a variety of other materials.

The results of the telephone survey and visits are summarized as follows:

(1) Most of the surveyed shops that specialized in ceramics worked primarily with alumina,

although most of them claimed some experience with other ceramics. Some of the shops

also worked with composites, cermets, and difficult-to-machine metals.

(2) Silicon nitride and silicon carbide give the most problems in machining. This, of course,

may be due to the fact that these are relatively new materials and experience is still limited.

(3) The shops surveyed used diamond grinding almost exclusively. Several had also set up

ultrasonic abrasive machining equipment. Most of the other techniques previously

described like laser cutting, single-point diamond turning, waterjet cutting, etc., were not

used. They do not plan to use these techniques in the future because of the extra capital

21
Table 6. QUESTIONNAIRE USED IN OBTAINING INFORMATION

FROM CERAMIC MACHINING INDUSTRY

1 . Do you machine just ceramics or other materials?

Alumina Metals

Silicon carbide Composites

Silicon Nitride Superalloys

2. What types of machining do you do on ceramics?

Diamond grind Other:

Ultrasonic

Thermal

3. Do you develop your own techniques; if not, where does the technology come from?

Material supplier

Wheel supplier

Technical literature

Other

4. Do you use special equipment or standards?

5. Do you work from prints supplied by customers?

6. What do you see are major technical problems that need to be resolved?

Machinability data

New techniques for more rapid machining

Improved equipment

NDE and quality control

7. What do you see are the critical factors in ceramic machining?

Operator skill

Machine quality

Properties of wheel

Fluid

8. What types of ceramic materials give biggest problems?

22
costs. However, if there was new equipment that substantially reduced costs, they would

have to acquire it to maintain their own competitive position.

(4) The shops develop their own techniques for each material with which they have not had

previous experience. They claim that help is sometimes available from wheel suppliers,

but that help is limited. They rarely go to the material supplier or inspect the technical

literature.

(5) Machinability data are rare for ceramics. The shops develop their own. Most shops

cited this as the major problem area. There are no established standards and no central sources of information pertaining to the grinding of ceramics. Each task must be handled

on a case-by-case basis. Their approach is very conservative; minimal removal rates are used to avoid surface damage and breakage. The conservative approach and the lack of an appropriate source of information greatly increases the cost of machining.

(6) There is a lack of information on damage measurement and control. This contributes to

the costly conservative approach used in machining. Also, because of the sensitivity of

ceramics to chipping and fracture, there is an abnormal amount of waste. New methods

to hold, grip, and transport ceramic parts are needed.

(7) Almost all shops cited operator skill as the critical factor in machining ceramics. Many

of their machinists had more than 15 years experience and had developed their own

special techniques. Unfortunately, this is costly and incompatible with large-volume

production.

3.2 Ceramic Machining Research in Industry

Some research in improved machining procedures is carried out in industry. This research is primarily concerned with the relation between machining parameters and product performance as indicated by strength, surface finish and residual damage. The research results are used to develop

procedures for machining of specific components. Within industry it was found that there were several

different approaches to the development and application of advanced ceramics. Although, these different

approaches cannot be generalized, the following patterns were noticed. The OEM's generally have a wait-and-see attitude. They have few incentives to pioneer in the development and use of ceramic

components and prefer to wait until they are economically feasible. They often rely on outside experts

to provide ceramic components and are primarily concerned with component evaluation rather than with

fabrication techniques. Component manufacturers (bearings, piston rings, valves, etc.), however, do have

a strong interest. They evaluate the potential market for ceramic components and move aggressively

when opportunities exist. The component manufacturers depend on the ceramic material suppliers to

supply either the final component or information on machining techniques. Some of the ceramic material

suppliers have set up their own machine shops, and others work with outside machine shops to promote

the development of production techniques for specific components.

Discussions on ceramic machining were also held with abrasive suppliers, diamond wheel

manufacturers and machine tool builders. All agreed that they were increasingly being asked questions

about ceramic machining, and that the current state-of-the-technology did not provide adequate answers. The abrasives manufacturers are quite active in understanding and improving their products. Thus, they

participate in ceramic machining research with the aim of demonstrating the capability of their products,

23
and work closely with the grinding wheel manufacturers to develop better grinding wheels. They also

work closely with OEM's and component manufacturers in setting up production processes.

The machine tool manufacturers feel that current equipment is adequate for present practice;

however, additional research will be needed to develop production machinery to make millions of ceramic

parts with adequate control of damage, finish and dimensions. It was agreed that if ceramics are to be

used extensively, large-volume production techniques with adequate on-line inspection would be required

to reduce dependence on operator skill.

3.3 Industrial Needs

It was the general consensus of the respondents that much could be done to improve the

conventional grinding process as applied to advanced ceramics. Industry trends in this direction were

identified and include creep-feed grinding, improved grinding wheels, improved knowledge of grinding

effects on performance, and more rigid machine tools. However, progress in advancing the ceramic

machining technology is very slow. Therefore, research and technology development programs should

be accelerated to keep pace with materials development efforts and industrial needs.

From all of the information that has been accumulated, several points emerge:

(1) Conventional grinding techniques will be used for the foreseeable future. Machinability

data are needed for ceramics for the optimization of the grinding process. Once removal

rate and damage criteria are established on an economic basis, potential advancements

can be explored on a rational basis.

(2) Damage introduced by machining is the overriding concern regardless of the machining

process used. Rapid machining is possible with current technology, but surfaces are left severely damaged and material properties deteriorate. Methods must be developed to measure surface damage in ceramics, and damage must be correlated with machining

parameters and material properties. Pre- and post-machining treatments may be useful

to remove residual machining damage. In-process inspection methods are needed for

controlling the machining damage.

(3) Certain techniques appear to have potential for increasing the removal rate while

minimizing damage. These include creep-feed grinding, and chemically assisted

processes. These should be explored in greater detail.

(4) A coordinated program is needed that provides inputs and cooperation from ceramic

component manufacturers, abrasive and wheel manufacturers, material suppliers and

fabrication specialists. Without a cooperative program, progress will be slow and

sporadic.

3.4 Current Government Programs

An informal survey was conducted to determine the extent of ceramic machining research

activities in Federal laboratories and government programs. This survey included database searches

(Defense Technical Information Work Unit Survey), phone calls to material departments of appropriate

government agencies and laboratories (DOD, DOE, NASA, Navy, Army, and Air Force research offices,

24
military laboratories and national laboratories), and a review of recent technical society bulletins which

announce research awards. This survey revealed a dearth of research and development in the area of

ceramic machining.

The National Science Foundation has supported a limited amount of research on machining of

glass and ceramics over recent years. Specific research projects include: temperature measurements in

grinding, laser processing of carbides and nitrides, ductile-regime grinding and ultra-precision machining

of glass components, and surface finishing of ceramics. These projects, however, are not coordinated

to achieve a specific overall program objective; but rather seek basic science information.

The Army Material Laboratory at Watertown Arsenal had a variety of programs which included

development of conventional and nonconventional machining methods and tooling for ceramics, electrical

discharge machining, waterjet cutting, laser cutting, damage characterization and control, and postmachining treatments. However, this program has now been discontinued.

The basic research offices of the Navy (ONR), Air Force (AFOSR), and Army (ARO) are not

currently supporting research in ceramic machining; neither are the Navy and Air Force materials

laboratories. The Bureau of Mines, NASA, and most other government organizations do not have any

current programs on machining of ceramics.

3.5 Research Activities in Other Countries

Although the main thrust of this survey was to analyze the status of ceramic machining in the

United States, it was deemed necessary to inquire into the research activities in other countries. An

examination of recent papers published in the English language indicated that researchers in Japan,

Germany, and the United Kingdom are very active in research related to machining of advanced ceramics.

Publications from Japan are mostly concerned with the development of new and innovative machining

techniques, for example, in the areas of polishing and ductile-regime grinding. Those from Germany and

the United Kingdom mostly deal with the development of specialized machine tools and creep-feed

grinding. Based on this examination, it can be safely stated that the total number of papers related to

ceramic machining from Japan, Germany, and the United Kingdom exceeds those published by the

researchers in the United States. This, however, does not include papers published in non-English

language journals, which were not available for this assessment. A noted example of the activities in

Japan and Europe is the availability of two new machine tools from Japan and the United Kingdom for

grinding of advanced ceramics. Considering the research papers and the industrial activities related to ceramic machining outside the United States, it can be concluded that a coordinated research and

development program would be crucial to sustain the economic position of the United States in the Global

market of advanced ceramics.

3.6 Recent Publications

To more completely define the research issues for a coordinated program, an additional literature

search was conducted to examine certain areas in greater detail. Three areas were selected:

• Optimization of the Grinding Process,

• Chemically Assisted Machining, and

• Machining Damage and Detection.

25
This literature search included a variety of databases (Ceramic Abstracts, Patents, Compendex,

Engineering Index, and the National Technical Information Service) and covered a period of 20 years.

The search key words are listed in Table 7. A total of 741 usable abstracts were retrieved as applicable

to the three subjects listed above. The distribution of abstracts among different machining methods is given in Table 8. Results of the literature search showed that abrasive machining accounted for 43

percent of the publications. Most of the publications concerned with nonabrasive methods applied to cutting as contrasted to machining for contour generation. This was particularly true for directed energy

beams. The survey also showed that chemical effects during machining have been investigated, but not

in an extensive or thorough manner. The results of the literature search are described in the following

three sections.

Table 7. LITERATURE SEARCH KEY WORDS

Process Material

Machining Ceramic

Finishing Aluminum Oxide

Grinding Silicon Nitride

Polishing Silicon Carbide

Cutting Zirconium Oxide

Honing Boron Carbide

Lapping Silicon Dioxide

Alumina

Table 8. DISTRIBUTION OF ABSTRACTS RETRIEVED ON MACHINING METHODS

Abrasive Methods (318) Nonabrasive Methods (105)

Grinding (190) Electrical Discharge Machining (28)

Honing (9) Chemical and Electrochemical

Lapping and Polishing (34) Machining (12)

Ultrasonic Machining (30) Laser Beam Cutting (55)

Liquid Abrasive Jet Cutting (16) Electron Beam and Ion Beam Cutting (8)

Turning and Cutting (39) Friction Cutting and Microwave

Cutting (2)

Combined Methods (25)

Chemical Polishing (15) NDE and OC (156)

Electrochemical Grinding (5)

Thermally Assisted Turning (3) Materials and Misc. (137)

Mechanical-Electrical Discharge (1)

Chemical-Electrical Discharge (1)

26
4. OPTIMIZATION OF THE GRINDING PROCESS

The dominant position of grinding in machining of ceramics can be attributed to a combination

of several important characteristics. These include: great flexibility in generating different workpiece

geometries, relatively low-tool forces and tool wear, and low workpiece damage. These same

characteristics are also the basis for the important position that grinding holds in the machining of metals.

The other abrasive methods may exhibit some of these capabilities, but not all. Honing, lapping,

polishing, and ultrasonic methods produce very low damage and smooth surfaces. Intricate geometries

can be generated with ultrasonic machining. Honing and lapping are outstanding in their ability to

produce close tolerances. However, for all these methods under most conditions, removal rates are

relatively low compared to grinding. Only limited precision and control of geometry is possible with

liquid abrasive jet machining except in cutting thin sheets where excellent results can be obtained.

Otherwise, liquid abrasive jet machining is restricted mainly to cutoff operations. Thus, of these methods

grinding is the most broadly applicable to machining of ceramics, while the others are employed for

specific tasks to which they are uniquely suited.

The purpose of this section is to review the fundamental mechanisms of material removal

processes during grinding and analyze the effects of important parameters that must be optimized in a production process. The parameters that control the removal rate and damage formation are specifically

highlighted. Some recent advances in grinding are discussed and recommendations are made on important

directions for research in grinding of advanced ceramics.

4.1 Fundamentals of Abrasive Removal

The abrasive methods, including grinding, all employ hard particles to remove material from the

workpiece. Forces applied to the abrasive particle (largely inertial in the case of ultrasonic impact and

liquid abrasive jet machining) cause it to penetrate and remove material from the surface by some

combination of chip formation, fatigue and fracture processes. The details of the process are determined

by the nature of the abrasive particle (shape, strength, size, etc.), the properties of the workpiece, the

interaction forces between the particle and workpiece, the time dependence of these forces, and the nature

of the surrounding environment. The physics and chemistry involved are complex and only understood

on the basis of greatly simplified models.

The simplest model of material removal by abrasion considers the penetration of the tip of a

single hard particle into the surface of the workpiece (Figure 11). As penetration occurs in a brittle

material, elastic displacement is followed by plastic flow and fracture. Fracture is considered to occur

only after a threshold load is applied to the particle. According to Evans and Marshall [1981] the

threshold load, P*, is given as:

P* = f (Kc4/H3

) f(E/H) (1)

where Kc is the fracture toughness; H, the hardness; and E, the elastic modulus of the workpiece material.

The function f(E/H) and the dimensionless constant f depend on the type of crack, shown in Figure 1 1

.

For most brittle materials the threshold loads are quite small. Machining at loads, or correspondingly

at particle penetration depths, less than the threshold is generally referred to as ductile-regime grinding

[Bifano et al. 1991]. The critical depth of cut for silicon nitride and silicon carbide is approximately 100

nm [Bifano and Fawcett 1991, and Mizutani et al. 1990]. The technology of ductile-regime grinding is

27
MEDIAN CRACKS-"*'

Figure 11. Lateral fracture mechanism [Evans and Marshall 1981].

still under development and is highly specialized in nature. To control the particle penetration depth, the

machines employed must be of the highest precision and rigidity. Also, a close temperature control is required.

Lateral, median and radial cracks are generated during the abrasive interaction, but removal of

material is considered to result principally from the extension of lateral cracks along the path of the

abrasive particle. Based on a model for lateral crack extension, Evans and Marshall [1981] have

developed the following relation for the volume of material removed by an abrasive particle, in a grooving process:

P„

V=c*3 (E/H)4/5 (2)

|£ 1/2 £[5/8

where Pn is the peak normal load; ^ the length of the groove; and a3 a material independent constant.

This equation has been shown to be in reasonable agreement with experiments for a relatively wide range

of different materials, demonstrating the general applicability of the concepts involved.

The Evans and Marshall model can be applied to honing and polishing, where material removal

is primarily the result of translating particles with respect to the surface to produce grooves. In lapping,

the motion of the particles is considered to be predominantly rolling rather than sliding [Indge 1990].

Rolling of the irregular particles is accompanied by repeated indentation of the surface. Fracture,

particularly at adjacent indentations is the mode of material removal. Similarly, indentation by the impact

of energetic particles is the primary mode of removal in ultrasonic machining and liquid abrasive jet

28
machining. Removal rate is a function of particle velocity, shape, mass, and attack angle, with the

highest removal rate being at normal incidence for brittle materials.

Although lateral cracks are the most important with respect to material removal, median and

radial cracks, which are normal to the surface, are the most critical in terms of strength limiting damage.

The dependence on load, hardness, and toughness which applies to the extension of lateral cracks is similar for radial and median cracks [Evans and Marshall 1981]. Thus, rapid material removal achieved

by large lateral crack extension will also result in larger and more damaging median and radial cracks.

The low thermal conductivity of some ceramics can also contribute to an increased susceptibility

to damage. Thermal stresses associated with high local temperatures may lead to larger cracks. A dull

and rounded grit which slides and deforms the surface without removing appreciable material is a major

source of frictional heating and increased temperatures. Excessive temperatures are not only damaging

to the workpiece but also to the abrasive particle, and hasten the degradation of the grinding fluid.

With complete control over abrasive particle size, shape, and contact dynamics, the workpiece

removal rate and damage would also, in principle, be under complete control. This is an ideal situation

that is not likely to be achieved in practice given the random shape and orientation of the abrasive grit,

uncertainties concerning the motion and forces applied to the grit, and variations in the workpiece

properties. Nevertheless, it is important to bear these principles in mind when considering abrasive

machining processes and the possibilities of their improvement. It might also be noted that these

principles lead to the set of rules of which most experienced practitioners are aware: maximum removal

rate with minimum associated damage is achieved by employing grit that is sharp, of the narrowest

possible size distribution, and in the highest concentration consistent with the removal of swarf.

4.2 Elements of the Grinding System

Having given a brief discussion of the physical basis of the removal process, the following

discussion will provide a short overview regarding the practical aspects of grinding. Greater detail is available in several text books [Metzger 1986 and Malkin 1989] and handbook articles [Davis 1989,

Drozda and Wick 1983, and King 1985]. As mentioned previously, it is convenient to consider the

grinding process in terms of the five elements of the system: machine tool, grinding wheel, truing and

dressing, grinding fluid, and workpiece. The first four of the elements will be discussed in this section.

References to the workpiece, specifically advanced ceramics components, is made throughout this report.

4.2.1 Machine Tool

Machines used to grind ceramics are generally the same as those employed for metals. For

application to ceramics one of the most important machine characteristics is that of rigidity. Because of

the greater hardness of ceramics, the normal force in grinding is usually higher than that for metals.

Thus, to minimize deflections, high rigidity of the spindle, table and feed mechanisms is required.

Freedom from vibration and precise control over all machine motions is also necessary in order to avoid

excursions in machining forces which are likely to increase the level of workpiece damage. Precise

control is also required by the fact that the final depth of cut may need to be very small to minimize

damage and produce a smooth surface. In general, a machine tool with computer numerical control

(CNC) is preferred. The more advanced machine shops specializing in the machining of ceramics are generally aware of these machine requirements, but other shops often are not.

29
4.2.2 Grinding Wheel

The grinding wheel is in many respects the most important element in the grinding of advanced

ceramics. The wheel consists of the grit, bonding medium, and perhaps a hub or other means of

supporting the grit/bond composite structure. The grit must be sufficiently hard and strong to penetrate

the workpiece and withstand the interaction forces during chip removal and must also have a high

resistance to the thermal and chemical degradative processes encountered during grinding. The function

of the bond is to hold the grit in a more-or-less rigid manner with the grit extending out from the bond

sufficiently to engage the workpiece. The concentration of grit should be a maximum, consistent with

the removal of swarf and other wheel characteristics. Some important bond properties are strength,

hardness, elastic modulus, resistance to thermal and chemical degradation, thermal conductivity, wear

resistance and dressability. Porosity and sometimes solid lubricants are incorporated in the bond structure

to achieve desired performance.

As noted earlier, diamond is the principal abrasive employed in the grinding of advanced

ceramics. Diamond is anisotropic in strength, showing cleavage on {111} planes and reduced wear

resistance on certain other planes and directions [Field 1979]. A major source of weakness or friability

is the presence of flaws and grain boundaries. Abrasive grit may be either natural or synthetic [Metzger

1986]. Grit prepared from natural diamond is crushed and sieved. Each particle is typically a single

crystal, irregular in shape and quite tough. Synthetic diamonds are prepared by a quasi-static, hightemperature, high-pressure process, or explosively. High-temperature, high-pressure synthesized

diamonds may be either monocrystalline or polycrystalline. Explosively formed diamond is polycrystalline with an extremely small microcrystalline grain size. Polycrystalline grit shows

considerably greater friability than monocrystalline grit. Friability is an extremely important property

in abrasive grit [Metzger 1986] since it is through this mechanism that the self-sharpening characteristics are obtained. Abrasive grit is available both uncoated and coated with metal, usually nickel or copper.

Metal coating improves retention in the bond, may hold friable grit together, and acts as a heat sink

preventing thermal damage to the bond.

In general, there are four different bonding systems used for superabrasive grinding wheels.

These are: vitreous, resin bond, metal bond, and plated. Each bond system has its own characteristics

which influences performance and application [Subramanian 1988]. Vitreous wheels are prepared by

mixing abrasive particles with clay, feldspar, frit and usually other additions, and firing at elevated

temperatures. Vitreous bond diamond wheels are a more recent development [Cooley and Wapler 1984]

and have demonstrated the ability to remove material at very high rates [Hoshimoto et al. 1984]. The

excellent truing properties and ease of dressing are also distinct advantages of the vitreous bond. Resin

bond wheels typically employ a thermosetting phenolic resin. Resin bonds hold the abrasive in a

relatively resilient matrix and can result in less workpiece damage. For this reason resin bond wheels

are the most widely used in grinding advanced ceramics. Metal bond wheels are formulated by sintering

the abrasive with powdered metals, principally bronze alloys. Metal bond wheels are often used in cut- off applications. Because of their high strength, metal bond wheels can be quite thin; this minimizes kerf

losses. Plated wheels are prepared by suspending abrasive particles in an electroplating or electrolessplating solution and depositing a layer on the wheel blank. Plated wheels of small dimensions are often

used to drill holes or machine intricate shapes.

Not only are there wide differences in properties associated with different bond classes but also

significant differences exist within each class. Each wheel manufacturer has its own proprietary

formulations and may offer many different variants of each bond. Combining the different bonding media

30
with the several different types of diamond grit available, there are many dozens of different wheels from

which to choose. Wheel manufactures may even provide custom formulations to meet a given customer's

requirements. A standard ANSI code is used to identify abrasive type, size, hardness grade,

concentration, bond type and other characteristics. However, there is no standard regarding performance

or application; and these features depend greatly on each manufacturers formulation.

In developing a wheel for application to advanced ceramics [Kuroshima et al. 1985 and Sugishita

et al. 1991] it is necessary to optimize the bond and grit components with respect to the properties of the

workpiece, required finish, tolerance, machine tool, grinding fluid, and the type of grinding operation,

e.g., internal, external, etc. An overriding consideration is, of course, the overall strength and safety

of the wheel. All of the foregoing factors result in a very large matrix of properties to consider. In

practice, once wheel safety is established, emphasis is usually given to providing required finish,

dimensional tolerances, and avoiding obvious forms of workpiece damage such as chipping and visible

fracture. This is largely accomplished by selecting an appropriate bond type, grit size, and adjusting feed

rates. Because of the relatively high rates of wheel wear experienced when grinding advanced ceramics

and the high cost of diamond wheels, minimizing wheel wear becomes an extremely important issue.

Wheel wear occurs by several mechanisms: attritive wear of the diamond grit, grit fracture, grit

pullout, and fracture or wear of the bond phase itself. Here, attritive wear of the grit refers to

mechanical, thermal, chemical, or a combination of wear processes that result in relatively smooth

rounding of the grit rather than larger scale fracture which leaves sharp points and edges. With advanced

ceramics, attritive wear of diamond is relatively rapid, which dulls the grit and results in workpiece

damage and poor grinding performance. It is for this reason that it is advantageous to employ friable

polycrystalline diamond instead of tough monocrystalline grit. For softer ceramics, for example glass,

for which attritive wear is small, the tougher monocrystalline grit are preferred.

Although the self-sharpening qualities of friable grit are an important advantage, the concentration

of exposed grit is continuously diminished, effectively reducing the available abrasive points. Here,

compensating wear of the bond would expose fresh grit and maintain the concentration. This can be

achieved to some extent with resin and vitreous bonds and is an important factor in bond formulation and

choice.

4.2.3 Truing and Dressing

Wheel truing and dressing are very important elements in the grinding operation. The term truing

refers to the process which is used to obtain and maintain a desired wheel profile geometry and

concentricity. Dressing is the process by which fresh and sharp abrasive grit are exposed. It is accomplished mainly by removing a small amount of bond, although dulled grit may also be removed.

The dressing of a used wheel may only involve the removal of adhered workpiece or decomposed

grinding fluid materials. The distinction between the terms "truing" and "dressing" is not always clear

and correctly made. In some cases the term dressing refers to both processes. Also, some truing

methods provide a dressed surface without additional treatment. The very properties that enable the

grinding of advanced ceramics are also those that tend to make diamond grinding wheels difficult to true

and dress. While truing and dressing of conventional aluminum oxide and silicon carbide grinding wheels

is often done with a single or multiple point diamond tool, this is not possible with diamond wheels.

31
Truing of diamond wheels can be accomplished by a number of methods [Green 1985, and

Metzger 1986]. One of the most widely used methods is brake truing. The brake truing device consists

of a vitreous aluminum oxide or silicon carbide wheel which is mounted on a spindle containing a braking

mechanism. With the truing wheel held against the rotating diamond wheel, braking results in sliding

as well as rolling, so that a grinding action takes place between the two wheels. Wear of the truing wheel

is, of course, rapid. Better results are said to be obtained by employing a motorized truing wheel which

is driven faster than the diamond wheel [Metzger 1986].

Crush truing can be employed with vitreous and recently developed crushable metal bond wheels

[Green 1985, and Metzger 1985]. In this method a freely turning wheel of mild steel is held against the

rotating diamond wheel with sufficient force to crush the surface of the diamond wheel. This method has

been widely used to true and dress vitreous aluminum oxide and silicon carbide wheels.

Dressing is accomplished by holding a soft aluminum oxide stick against the wheel, thereby

wearing away the bond. Resin bond wheels may also be dressed by grinding a mild steel block which

produces long chips that cut away the bond [Metzger 1986]. Dressing by means of an abrasive fluid jet

has been demonstrated by Salje [1985]. Electrical discharge machining (EDM) [Kaiser 1977, and Shore

et al. 1991] and electrochemical machining (ECM) [Murata et al. 1985, Ohmori et al. 1991 and Suzuki

et al. 1991] have been employed in dressing metal bond wheels. The latter methods can be used for

continuous dressing during grinding. With this approach wheel dressing can be adjusted according to grit

wear and the maximum efficiency in grinding rate maintained. A schematic of the in-process wheel

dressing used by Murata et al. [1985] is shown in Figure 12.

Electro Conductive

Table Feed

Figure 12. Schematic of in-process wheel dressing by electrochemical machining [Murata et al. 1985].

32
The selection of an appropriate grinding wheel for metals applications is fairly well established

by each manufacturer, although there may be many trade-offs to consider. For example, rapid removal

rate is usually accompanied by higher wheel wear which in turn affects achievable precision of form and

surface finish. For application to advanced ceramics, recommendations regarding wheel type and

appropriate grinding parameters can be obtained from wheel manufacturers. Many of the shops queried

in this study stated that such recommendations have not been satisfactory. It was thought that this was

due to the sensitivity to different machine characteristics, workpiece properties, and operator practices.

It was suggested that a more successful approach to wheel selection was through the provision of a trial

wheel to the customer for evaluation. The customer would then report back to the wheel manufacturer

problems such as insufficient wheel wear life (G-ratio), incorrect surface roughness, a tendency towards

burning, or excessive workpiece chipping. The wheel manufacturer could then provide a different wheel

or make certain adjustments in the formulation, all based on knowledge about the relationship between

grinding characteristics and wheel properties gained from previous experience.

Many opportunities exist for enhanced performance in the grinding of advanced ceramics by

improving wheel properties. These include optimizing grit properties such as friability and shape,

matching grit and bond wear characteristics for application to specific advanced ceramic materials, and

developing improved truing and dressing methods. This could also include the optimization of bond

materials for continuous dressing methods. For example, the development of conductive resin bonds

would permit continuous EDM or ECM truing and dressing.

4.2.4 Grinding Fluid

The main functions of the grinding fluid are to control the temperature of the wheel and

workpiece, and carry the swarf away from the contact. In addition, the fluid may also lubricate the

wheel/workpiece contact, thereby reducing the friction force and also frictional heating. The lubrication

function is especially important where there is a tendency for the workpiece material to adhere to and

"load" the wheel. While serving these important functions, the grinding fluid must possess a number of

other characteristics. It must be stable and durable and not be hazardous to operating personnel and the

environment, and should not contribute to, or better, should protect the equipment from corrosion.

Using the most effective grinding fluid can allow higher wheel and table speeds, greater depths

of cut, increased wheel life, better surface finish, reduced subsurface damage, and improved dimensional

control. Clearly, the grinding fluid is an important element in the grinding process.

Grinding fluids used for metals can be classified into three types: mineral oils, soluble oils, and

chemical fluids. These will be described shortly. But, first, we must recognize that grinding fluids

contain various additives to improve their performance. Additives are used to reduce friction and wear

at the cutting interface, to retard chemical degradation of the fluid, and to protect the workpiece from

severe reactions with the fluid. Other types of additives are used as emulsifiers, foam inhibitors, and

bacteriocides.

Straight mineral oils and mineral oils plus polar and extreme pressure (EP) additives are usually

used in low speed grinding and metal cutting. Because of the higher heat capacity of water compared

to mineral oils, it is assumed that waterbase fluids perform better in controlling the grinding temperature.

Recent research, however, has shown that mineral oils are superior to waterbase fluids in performance, even at higher speeds [Osman and Malkin 1973, and Howes 1990]. To explain these results, we must

consider that approximately 50 percent of the heat is generated from sliding at the abrasive/workpiece

33
and abrasive/chip interfaces. The rest comes from deformation in the shear zone. Therefore, waterbase

fluids may be efficient in controlling the overall workpiece temperature by removing the heat from the

cutting zone. However, mineral oils and the additives will reduce friction and thus the heat generated

at the sliding interfaces [Mittal et al. 1984, Mould et al. 1972a and 1972b, and Wakashiro et al. 1990].

This action will have a direct effect in reducing the machining damage in the cutting zone. Furthermore,

the lubricating additives decrease the rate of wear of the abrasive grains [Duwell et al. 1988], and reduce

the adhesion tendency between the chips and the wheel.

Although mineral oils are superior to waterbase fluids, they are not generally used in the United

States because of fire and health related safety hazards, which would require special containment and

improved ventilation. The oil mist generated during high speed grinding can make the area around the

machine slippery and, therefore, hazardous. Also, inhalation of the hydrocarbon vapors could be

harmful. Disposal of the used mineral oils is costly. The alternative is to use soluble oils, which are emulsions of oil in water, where the fraction of water is generally much greater than that of oil. These

fluids contain additives as emulsifying agents, as well as polar and extreme pressure additives for friction

reduction. Soluble oils have the advantage of high heat capacity of water, as well as the lubricating

capability of the mineral oils and lubricous additives. One disadvantage of the soluble oils is their white

milky color, which obscures vision of the cutting zone.

Chemical fluids are mixtures of water and water soluble additives, such as polyglycols, amines,

alcohols, and phenols. These fluids are transparent but they do not, in general, have the lubricating

capability that either mineral oils or soluble oils have.

The high stresses and temperatures generated at the grit/workpiece contact provide favorable

conditions for chemical reactions which can exert a significant influence on the grinding process. The

reactions may involve the workpiece, the grinding wheel (both grit and binder), the grinding fluid and

the air atmosphere. Any pair or combination of several of these entities may participate in the reaction.

As an example, it is well known that diamond is not a satisfactory abrasive for grinding iron and ferrous

alloys unsaturated in carbon. The affinity of these materials for carbon is such that carbon atoms from

the diamond diffuse into workpiece resulting in a high wear rate of diamond. Cubic boron nitride (CBN),

however, is much more stable with respect to reaction with ferrous alloys and is an effective abrasive for

these materials. On the other hand, CBN presents an interesting example of the effect of the

environment, in this case the grinding fluid, on wear. CBN performs poorly in the presence of many

waterbase fluids, exhibiting wear rates many times greater than with mineral oil grinding fluids [Metzger

1986].

There are numerous examples [Malkin 1989] of the improvements that can be gained by selecting a fluid that through chemical means increases the capability to grind a material at higher rates. Sulfur

and chlorine additives are used in grinding fluids for metals. Other additives are used to grind glasses.

Similar examples are not available in the case of advanced ceramics, although in discussions with ceramic

machine shops, it was stated that some fluids were notably better than others.

4.3 Suggested Technical Approaches

Considering the widespread use of grinding and the fact that significant opportunities exist to improve the grinding technology for application to advanced ceramics, a focused research and

development effort could result in substantial economic benefits. Particularly, research efforts should

concentrate on obtaining machining data for grinding optimization, development of grinding fluids and

34
diamond wheels specifically designed for advanced ceramics, development of high-strength machinable

ceramics, and design changes to the machine tool. Some of these research areas are described in more

detail in Section 7. Certain fundamental mechanistic issues also need to be investigated; for example,

the mechanism of material removal involving multigrits as opposed to the present model that involves

only a single grit, the effect of other crack systems in addition to the lateral cracks, and the effect of

environment on crack extension. Research on machine tools should focus on design changes that would

allow further utilization of potentially important processes of creep-feed grinding [Konig et al. 1991] and

ductile-regime grinding [Mizutani et al. 1990]. Some new concepts of machine tool design [Salmon

1990] also need to be considered. Other issues related to chemical effects during grinding and detection

of machining damage are described in more detail in the following sections.

35

5. CHEMICALLY ASSISTED MACHINING

In the literature search only a few publications were found that considered chemical effects on

the abrasive machining of ceramics. Most of these publications were concerned with the drilling of rocks

and polishing of glass. The results reported in the publications were analyzed with respect to applicability

to ceramic machining, and are reported in this section. Based on the analysis of the published literature,

it is suggested that abrasive machining processes including grinding, liquid abrasive jet cutting, ultrasonic

machining, and single-point turning, may benefit from chemical assistance, introduced by the grinding

or cutting fluid.

In general, the fluid appears to play a stronger role in the process than just temperature and

friction control and swarf removal. The chemicals can affect the workpiece, the abrasives and their wear

rates, the transfer of material, the action of the swarf, the properties of the wheel, and other factors. The

effects may be either beneficial or harmful and each case must be independently investigated. Although

chemical techniques may have a series of beneficial effects, they can not be so aggressive as to endanger

the working environment, or add great complexity to ceramic machining processes.

5.1 Chemical Effects in Machining

Corrosion rates of advanced ceramics in liquids are reasonably rapid only for certain specific

combinations of materials and environments [Anonymous 1990]. Examples are phosphoric acid acting

on alumina, sulfuric acid and nitric acid on tungsten carbide, and sodium hydroxide and potassium

hydroxide on Si/SiC composites. In all these cases, temperatures near 1000°C are needed to achieve

sufficiently fast reaction rates. Chemical milling techniques have been developed for difficult to machine

metals, e.g., titanium. It is possible that such an approach would succeed for ceramics with particular

machining requirements.

Molten salts are known to be corrosive to ceramics [Jacobson 1986]. In studies of sintered SiC

it was found that thin films of Na2C03 (in C02 atmospheres), Na2S04 (in 02 atmospheres), and Na2S04 (in S03 atmospheres) all reacted significantly at temperatures of 1000°C. The mechanisms generally

involved silicate formation and dissolution in the molten salt. In studies of alumina, aqueous solutions

of sodium borate were found to be corrosive at 900°C [Scott et al. 1986]. Whether such reactions can be utilized in other ways at lower temperatures appropriate to chemically assisted machining is not clear;

further study is needed.

Yeomans and Page [1989] have reported on liquid metal attack of zirconia-toughened alumina,

sialon, and silicon nitride ceramics. In the latter material, grain boundary phases were particularly

susceptible. Liquid metals studied included iron-based and nickel-based alloys at temperatures in excess of 1000°C.

5.2 Chemical Effects in Drilling

Extensive studies of chemical effects on diamond drilling of stone have been reported [Cooper

1979, and Mills and Westwood 1980a]. It was found that the effect of liquids on drilling hard stone was

that of a reduction of diamond wear rather than a direct effect on the stone itself. In one study it was

found (Figure 13) that addition of 12 percent glycerine to water reduced the rate at which the drill

37
penetration rate decreased with drilling distance, and at the same time maintained a higher torque (i.e.,

friction coefficient) during drilling [Cooper 1979].

The effect of normal alcohols on diamond drilling of granite also has been studied by Cooper and

Berlie [1976]. They reported a significant improvement in drill penetration characteristics upon

alternating the substitution of a series of alcohols for water as the drilling fluid. Figure 14 shows the

observed reduction in loss of penetration rate for the drill. The results were interpreted to reflect a reduction in wear of the diamonds in the drill bit.

r

4 AS

em

«* 1 cc

1

f S T

r \

b c

-——12 pet Glycerine —••— Water

600 lb thrust

1,000 rpm

27.0 in.

JL3 4 5 6

DISTANCE DRILLED, in

(a)

10

0.48

§ -36

o

LlI O

<-> .24

o

£ .12

' '

'

' 1 1 1 1 1 1 1 1 11

12 pet Glycerine

Water

600 lb thrust

1,000 rpm

*—»-.—

till i i i i i i i i i i 4 6 8 10

DISTANCE DRILLED, in

12 14

(b)

Figure 13. Effect of environment on (a) drilling penetration rate in quartz, and (b) friction coefficient

derived from torque on the drill [Cooper 1979].

38
DRILLED LENGTH, m

Figure 14. Effect of environment on drilling rate in granite, alternating water and a series of alcohols

[Cooper and Berlie 1976].

PENETRATION, in

Figure 15. Effect of environment on drilling rate in granite [Mills and Westwood 1980a].

39
Mills and Westwood [1980a] have reported a study of the effect of one chemical additive on

diamond drilling in granite. The effect of using a commercial cationic surfactant in aqueous solution is shown in Figure 15. It is seen that the additive did improve the penetration rate of the diamond drill

compared to that observed with water as a drilling fluid. In an earlier study of another additive [Mills

and Westwood 1977], the authors also reported a significantly improved diamond drill penetration rate

in granite as shown in Figure 16.

Westerly Granite

PENETRATION (cm)

Figure 16. Effect of environment (dodecyl trimethyl ammonium bromide, DTAB) on drilling rate in

granite [Mills and Westwood 1977].

Mills and Westwood [1980b] have also reported chemo-mechanical effects on diamond drills

applied to other types of hard rock. Figure 17 shows such a comparison using a 0.25 percent solution

of a cationic surfactant (FBH) in water. While the initial drilling rates were similar, the surfactant

extended the life of the bit by a factor of 2 to 3 compared to water, in which the diamond wore more

rapidly.

Similar effects to these for drilling of rock have been reported for drilling an alumina ceramic.

Swain et al. [1975] found a good correlation between maximum drilling rate with diamond core drills and

the isoelectric point (no net surface charge) of the surfaces and various alcohols and aqueous fluids. As

shown in Figure 18, in all three cases the drilling rate maximum corresponded to the isoelectric point.

They also measured surface hardness by microindentation and found that it was maximum at the

isoelectric points. The authors suggested that cutting occurred by a brittle process that was most efficient

at maximum surface hardness.

Mills and Westwood [1980a] and Westwood et al. [1981] have reviewed and discussed the

apparent diversity of findings in the literature on the influence of chemistry on mechanical behavior and

on drilling behavior. They point out the complexity of the processes involved. While one mechanism,

40
2.0

1.5<»- \

Rhyolite Porphyry: 2.375 in. o.d. surface-set bit

0.25 w/o- FBH in Water

Change

to water

PENETRATION (in.)

Figure 17. Penetration rate for rock drilling using water and water plus 0.25% FBH [Mills and

Westwood 1980b].

absorption-induced changes in surface energy, may occur in special cases, it cannot be the general

mechanism because of kinetic requirements. They also state that another mechanism, the effect of

adsorbed surface species on dislocation mobility in a solid being mechanically stressed, cannot be

generally invoked; and they reference experimental evidence for support. They concluded that for the case of diamond drilling in hard rock, it appears that the controlling mechanism is wear of the diamond

grit.

In drilling soft ceramics, however, environmental effects on the flow-fracture behavior of the

solid can be controlling. Figure 19 shows some results of Westwood et al. [1981] on drilling calcite, a

relatively soft mineral. The initial portion of the drilling curve shows a small decease in the drill

penetration rate due to drill wear occurring in the butyl alcohol environment. Upon adding 10% water

to the alcohol, drill penetration rate increased abruptly, indicating an effect of the environment on the

calcite surface behavior.

While Westwood and co-workers state that substantial improvements in drilling, up to a factor

of five or more, can be expected, they emphasize that this is only true if the proper conditions are

41
5 6 7 8 9 10 11 12 pH VALUE

Figure 18. Effect of pH on (a) zeta potential, and (b) drill rate in alumina for three solutions [Swain et

al. 1975].

present. Those conditions involve drill speed, drill bit geometry, additive concentration, and material

type and hardness, among other factors, since the chemo-mechanical effects are strongly influenced by

adsorption kinetics. They state that under unsuitable conditions, there may be no observable effects.

In another view of the subject, Cuthrell [1979] has discussed the effects of chemical environments

on ceramics, glass, and minerals during cutting. He states that hydrogen effects are crucial, and that

42
5

E 4

S5O 3 H

'

i

W

1

25 50 75 100 125 150

TIME (sec)

Figure 19. Drilling penetration rate for calcite using two solutions [Westwood et al. 1981].

hydrogen availability is dependent on the type of binding in the liquid. It is not yet clear how applicable

this explanation may be.

5.3 Chemical Effects in Polishing

Abrasive polishing of surfaces utilizes fine grit abrasive particles and low contact pressures in

order to control cutting depths to small amounts for the desired high surface smoothness. As a result,

low material removal rates are obtained. These conditions are such that chemical effects from the

polishing environment can be significant.

Vora et al. [1982] have studied the process of obtaining highly smooth surfaces of silicon nitride

using two oxides of iron. Polishing was done using dry powders at moderately high pressures to create

high contact temperatures, inducing reactions at the contact spots. Surface roughness (peak-to-valley) less

than 20 nm was obtained. The polished surface contained silicon oxynitride and traces of iron. While

polishing rates were small, up to 1.6 jiim/h, surface scratching was avoided because of the relative

softness of the oxide particles compared to the silicon nitride.

Rostoker [1990] has reported the development of a proprietary chemical additive that leads to

increased polishing rates on fused silica using small (3 jum) alumina grit, while retaining the usual good

surface finish. The importance of using small size grit is related to residual workpiece damage, and to surface finish.

Yasunaga et al. [1979] reported on the differences measured in polishing silicon surfaces with

various soft abrasives in both dry and wet (water) environments. As shown in Table 9, the relative

43
difference in polishing rate for the two environments can range from 20% to a factor of 8 times

depending on the system. In all cases the material removal rate is low, but the surface finish quality was

stated to be high.

Sugita [1986] has reported a vibratory method of polishing MgO crystals in water that leads to

very good surface finishes presumably through chemo-mechanical effects on the surface. Sugita et al. [1979] previously described the results of a study of polishing of sapphire (0001) crystal surfaces in water

without abrasive particles by the use of a rotating alumina disk. The friction of the contact caused

removal of a hydrated layer on the sapphire surface without appreciable damage to the underlying

sapphire. This method produced fine surface finishes in the order of 20 nm rms.

Table 9. MEASURED POLISHING RATES FOR (1 1 1)

SURFACE OF SILICON UNDER DIFFERENT CONDITIONS

[Yasunaga et al. 1979]

Powder Grain Size Polisher Environment Polishing Rate

(jxm/h)

BaC03

3 - 10

Cloth Dry 1.16

Cloth Wet 0.17

2-4

Bakelite Dry 2.60

Bakelite Wet 0.30

Fe304 10-20

Cloth Dry 0.68

Cloth Wet 0.28 j

CaC03

4-7 Cloth Dry 0.59

2 - 3 Cloth Dry 0.97

Si02

2 - 3 Bakelite Wet 1.30

0.5

Cloth Dry 1.00

Cloth Wet 0.85

0.01 - 0.02 Cloth Dry 1.91

Cloth Wet 1.05

Ce02 2-3 Bakelite Wet 0.91

Diamond 1 Cloth Dry 2.2-4.2

44
Glass surfaces are well known to be sensitive to the polishing environment, and the results of

Cook [1990] shown in Table 10 illustrate this. The polishing rate shown varies by over a factor of 50

times depending on the cation species in the polishing compound. Ce4+ cations produce the highest rates

in the conditions used. Cook discusses the role of chemistry in mechanical polishing of glass, and notes

that the highest polishing rates found in two separate studies corresponded to the isoelectric pH point of

the surface; Figure 20 shows one set of results.

ISOELECTRIC pH

Figure 20. Polishing rates for glass using different cationic species, showing the isoelectric points in each

case [Cook 1990].

The results obtained by Bifano et al. [1992] and Gollini and Jacobs [1991] clearly show the strong

influence of the chemical environment on polishing of glass. For example, Gollini and Jacobs have

shown that the process of material removal in polishing of glass can be changed from a brittle process

to a ductile process by changing the chemistry of the carrier fluid. In this study, alcohols of various

chain lengths and other fluids such as ethylene glycol, glycerol, and aniline were used.

Duwell et al. [1985] have drawn attention to the importance of chemical effects in abrasive

polishing of titanium. By adding phosphate compounds to the water solution used to flood the surface,

they found improved performance with both alumina and silicon carbide abrasives. They proposed that

cationic adsorption on the SiC grit surfaces probably explained the better performance of that abrasive

in their tests.

45
Table 10. POLISHING RATE DATA FOR GLASS BY SELECTED POLISHING COMPOUNDS

[Cook 1990]

Cations

and

Coordination

No.

R-0 Bond

Strength

Polishing

Rate*

( n m/f*vr*1 p\

Polishing

Rate**

( iiVC\lc\]c\p*\

Ce4+ » 61 4 4 9 2 98

Cr3+ 6 41 8 3 5 0 59

Th4+ 8 70.1

Fe3+

x w 6 29 5 3 1

Ga3+ 6 39.8

Zr* + 8 62.4 2.8 1.62

Ti4+

J. 1 6 71 1 3 0 1 31

Sn4+ 6 41.8 2.5 0.61

Ni2+ 6 17.8 1.2

Zn2+ 4 31.5 0.9

Hf+ 8 63.1

In

3+ 6 33.6

Mn4+ 6 37 3 1.5

Fe2+

J. V 6 19 3

Al3+ 6 63 2 1x • u8 0 19

Y

3+ 6 72 5

Al3+ 6 63.2

Sc3+ 6 72.6

Ce3+ 6 68.3

La3+ 6 66.7 0.075

Si02 4 102.5 0.01

Note: Consult reference for additional explanation

* Data from Kaller, see [Cook 1990]

** Data from Silvernail, see [Cook 1990]

46
5.4 Chemically Assisted Grinding

In concept it is possible to have significant chemical interactions between the local environment

in a grinding system and any or all of three elements: the workpiece, the grinding wheel matrix, and the

diamond abrasive. As an example, Bifano et al. [1988] have reported a dramatic increase in the

brittleness of the grinding process (i.e., a decrease in the allowable depth of cut for ductile grinding) for

both fused silica and quartz but much less an effect for silicon, upon substituting alcohol for water as the

grinding fluid. Beneficial effects on diamond wear during rock drilling were noted earlier. Other studies

have shown the advantage of chemical attack on the wheel bond matrix as an effective means of

continuous dressing of the wheel for optimum relief of the diamond grains [Murata et al. 1985].

A possible approach for introducing chemical assistance in grinding is through chemical additives

in the grinding fluid. We found no published research directed toward the development of specific

grinding fluids for advanced ceramic materials. Grinding fluids currently used for ceramics were

apparently designed for grinding of metals and, therefore, have not been optimized for best performance

with ceramics.

By utilizing chemically reactive compounds in the grinding fluid, it may be possible to control

the cutting process at the abrasive/workpiece interface. Ceramic materials, despite the commonly held

view, are not chemically inert, particularly when subjected to mechanical stress. The term

"tribochemistry" is used to describe chemical reactions in the presence of mechanical forces or deformation [Heinicke 1984a]. This phenomenon has been used for many years in the processing of

ceramic powders [Jimbo 1984, Heinecke 1984b]. For example, the reactivity of silicon nitride with water

vapor increases severalfold with mechanical action and yields silicon oxide and ammonia [Volante et al. 1989]. Similar reactions have been observed in tribological studies where silicon nitride slides against

silicon nitride [Fischer and Tomizawa 1985, and Tomizawa and Fischer 1987]. Tribochemical reactions

between water and alumina, and also water and silicon carbide, have been reported [Gates and Hsu 1991,

Dong et al. 1991, and Hegemann et al. 1988].

Recent studies have shown that friction and wear of ceramics are altered by chemical compounds

that are normally used in lubricants [Jahanmir and Fischer 1988, Studt 1987, and Gates and Hsu 1991].

It is also important to note that the friction coefficient of ceramics, unlike metals, is considerably reduced

by pure hydrocarbon oils that do not contain surfactants [Jahanmir and Fischer 1988, and Deckman et

al. 1991]. In a recent study [Gates and Hsu 1991], several chemical compounds were evaluated for their

potential use as lubricant additives. Table 11 lists these additives and summarizes the results of friction

tests. The results clearly show that most compounds are chemically active toward silicon nitride. Polar

additives (e.g., oleic acid), oxygenated compounds (e.g., glycol), and phosphates (e.g., zinc dialkyl

dithiophosphate), decrease both the friction coefficient and the wear rate of silicon nitride. Others, such

as chlorinated and sulphurized compounds, act as pro-wear agents. These compounds are commonly used

as lubricant additives and as additives in some grinding fluids used with metals [O'Brien 1984].

Generally speaking, the polar and the oxygenated compounds adsorb on the contacting surfaces either

by physical or chemical adsorption process [Jahanmir and Beltzer 1986a and 1986b]. But phosphates and

sulphurized compounds chemically react with the sliding surfaces in the contacting zone, to produce thin

films (less than 0.1 /xm) on the surface [Jahanmir 1987]. Both the adsorbed films and the reacted films can reduce adhesion and friction at the cutting interface, and thereby control the cutting process [Zhang

etal. 1991].

47
Table 11. FRICTION AND WEAR DATA ON SELECTED MODEL COMPOUNDS

[Gates and Hsu 1991]

*

Dia Increase Friction Friction WEAR bCAR Film in

1 Chemical Compound Above Hz, mm COEFF. Type Appearance Scar? NONE 0.267 0.113 smooth no

Metal Salicylate (overbased) 0.032 0.103 IV smooth yes Oleic Acid 0.034 0.076 II smooth yes—plastic Clvcol A (polypropylene-diol) [1000 MW] 0.042 > antiwear 0.077 II smooth yes—plastic Glycol B (polvpropvlene-triol) [BOO MW] 0.072 0.078 II smooth yes—plastic

Glycol C (polvelhvlene-diol) [400 MW] 0.077 ' 0.063 II smooth yes—plastic Glycol A (polvpropvlene-diol) [400 MW] 0.279 0.130 III smooth no

2. 6-ditertiarybutlv-para-cresol 0.281 0.123 I smooth no

Ethyl Stearate 0.316 0.120 I smooth no

Hif^h Erucic Acid Rapeseed Oil 0.33

1

0.120 III smooth no

Polvol Ester 0.369 Drowtar po 0.134 I smooth no

Fatty and Synthetic Esters 0.396 0.111 III smooth no

Alkvl ZnDP A 0.024 0.088 II smooth yes—Huid

Alkvl ZnDP B 0.081 II smooth yes—Huid

Alkvl/Aryl Phosphonate 0.027 0.085 II smooth yes—plastic Aryl Phosphate A 0.036 I 0.081 II smooth yes—plastic Arvl Phosphate B 0.053

'

\ antiwear 0.090 II smooth yes—plastic SbDP 0.055

| 0.077 II blotchy yes—plastic Alkvl Phosphate 0.055 1 0.080 II smooth yes—plastic Aryl Phosphite 0.064 0.074 II smooth yes—plastic Alkvl Phnnnhiie 0.074 0.086 II 311 1 VJV.J 1 1

1

vps nl;i^n< y i_ 3 L/idsnij AnH Phnsnnalp 0.099 0.076 II mi i tynf^nFrl vf*s—— nlsisrir

Chlorinated Paraffin 0.377 0.118 I smooth no

Chloro-sulfurized Lard Oil 0.382 0.147 III si. grooved no

Chlorinated Fatty Acid (FA) 0.426 prowear 0.113 III si. grooved no

Chlorinated FA Ester 0.431 1 0.127 III smooth no

High MW Substituted Imidazoline 0. 156 i 0.077 II smooth no

3

Succinamide 0.187

antiwear

0.131 III smooth no

Dinonyldiphenylamine 0.278 0.121 I smooth no

Metal Sulfonate A (overbased) 0.011

N 0.108 IV smooth yes Metal Phenate (overbased) 0.042 antiwear 0.096 II smooth yes Metal Sulfonate B (low base) 0.043

| 0.089 11 smooth yes OxyMoDP (sulfurized) 0.108 0.060 II smooth yes—plastic Antimony dialkvldithiocarbamate 0.260

J 0.114 I smooth no

Sulfurized Hydrocarbon 0.271 0.1 14 I smooth no

Methylene bis(dibutvldithiocarbamate) 0.277 0.125 I smooth no Organo Molybdenum Dithiocarbamate 0.308 0.072 11 blotchy yes—plastic Sulfur-Molybdenmum Compound 0.338 ' prowear 0.072 11 blotchy yes—plastic Sulfurized Fattv Oil 0.378 0.137 III smooth no

1 1 wt 7c in purified paraffin oil "Measured at (he end of the test

'Significant amounts of film outside the wear scar

Relative wear scar diameter

48
Studies on friction of diamond and diamond film [Jahanmir et al. 1989, Buckley 1981, and Gardos and

Soriano 1990] have shown that diamond, similar to ceramics, responds positively to hydrocarbon

lubricants and polar additives. Therefore, it may be possible to add chemical compounds, that either

adsorb or react with the ceramic surface in the cutting zone, to the grinding fluid. Furthermore, the same

additives may adsorb on the diamond abrasive particles to reduce friction and wear of the diamond, and

also reduce the tendency for the adhesion of the swarf to the grinding wheel.

5.5. Chemically Assisted Liquid Abrasive Jet Cutting

Chemical assistance could conveniently be provided through modification of the fluid in liquid

abrasive jet cutting to increase the cutting rates. Water soluble chemical species can be added to enhance

the cutting rate through effects that reduce wear of the abrasive particle cutting edges. Other additives

could assist in providing viscosity control of the fluid and to improve suspension characteristics of the

abrasive particles. Duran [1984] has reported on the use of a dilute solution of alkylaryl polyether

alcohol in octyl alcohol for use with boron carbide or silicon carbide abrasive slurry in cutting boron

carbide. The additive served to assist in maintaining the abrasive grit suspension by acting as a wetting

agent without altering the fluid viscosity or the flow characteristics associated with the jet.

5.6 Chemically Assisted Ultrasonic Machining

As shown in Figure 6, the contact zone in this process involves the tool, the workpiece surface,

the free abrasive grit, and a carrier fluid. The ultrasonic abrasive machining method would appear to lend itself directly to chemical reactions between the fluid and either the workpiece or tool surfaces, either

to enhance machinability or to extend tool life. If attempts are made to retain the abrasives in the cutting

region by design, perhaps to increase efficiency, then wear and fracture of the grit to smaller grit sizes

may be a concern. In this case, chemical and environmental effects could be useful to control the

processes of grit wear and fracture; and thereby, enhance the machining process. Changes in fluid

properties such as viscosity, and the ability to suspend solids, are also advantageous.

5.7 Chemically Assisted Single-Point Diamond Turning

Wear of diamond single crystals is generally controlled by cracking and chipping and is influenced by the chemical environments and thermal conditions during single-point diamond turning

[Field 1979, and Wilks and Wilks 1979]. It was pointed out earlier that application of single-point

diamond turning to advanced ceramics requires special attention to the wear of the diamond tool. By

controlling the chemical and thermal environments during cutting, through properly designed cutting

fluids, it may be possible to reduce the tool wear and optimize the removal process. The following

examples are cited in support of this argument.

Biffano et al. [1988] and Blackely and Scattergood [1992] have investigated single-point diamond

turning of germanium using water as a cutting fluid. Figure 21 shows results from several experiments

relating cutting feed rate and chip thickness. In this case, it was possible to use larger depths of cut and

feed rates under dry conditions than with water as an environment. Gielisee et al. [1974] have reported

that alcohols affect the rates of single-point diamond turning of three types of alumina. The force levels

required and the amount of material removed increased as the alcohol chain length was increased. Singlepoint diamond scratching experiments have also been used to study effects of liquid environment [Van

Groenou et al. 1979] on different materials including alumina, magnesia, and glass. It was found that

there was little effect on the forces and the energy for grooving by the diamond point. However, there

49
n

3

—

A Distilled water • No water

i

0 50 100 150 200

A (t/f) 2

Figure 21. Difference in single-point diamond turning of germanium under two conditions, where t =

chip thickness, f = feed rate, and A and C are constants [Blackely and Scattergood 1992].

was an effect on the damage created in the workpiece during scratching in the different liquids: n- alcohols, dimethyl formamide, dimethyl sulfoxide, and dodecyl trimethyl ammonium bromide.

5.8 Suggested Technical Approaches

The foregoing discussion and the review of the published literature confirms that enhancement

of chemical reactions at the interface between the tool and the ceramic workpiece is a feasible method

for controlling the material removal process. It is, therefore, suggested that research projects be focused

on elucidating the fundamental chemical reactions that may occur during abrasive processes such as grinding, polishing, ultrasonic machining, liquid abrasive jet cutting and single-point turning. The goal

of these investigations should be to develop recommendations for the selection of chemical compounds

to be added to the grinding and cutting fluids to increase the removal rate and decrease the residual

machining damage. It should be recognized, however, that the timescale of reactions is extremely

important. The chemical reactions should occur at timescales comparable to the removal rate to be

effective, but not so fast that aggressive attack of the machine tool components or the degradation of the

surface of the workpiece would occur.

50
6. MACHINING DAMAGE AND DETECTION

In this section the mechanisms of damage formation during grinding and other abrasive processes

are reviewed, and possible approaches that increase the efficiency of material removal and decrease the

extent of residual machining damage are discussed. Due considerations are given to nondestructive

methods for detecting machining damage and to relationships between damage and resulting material

properties.

6.1 Mechanisms of Damage Formation in Grinding

It is important to recall the processes of abrasive removal of materials as schematically shown

in Figure 22. The abrasive particle is forced to penetrate the workpiece as the workpiece and the particle are moved relative to each other and parallel to the surface. Depending on the contact conditions and

the mechanical properties of the workpiece, chips are generated by plastic flow and/or brittle fracture.

In materials capable of plastic deformation, for example metals, chips are formed by shear in a

ductile mode, Figure 22a. In materials that have a limited ductility, for example ceramics, chips are generally formed by a brittle fracture process, as depicted in Figure 22b. Whether the chips are formed

by brittle fracture or by plastic deformation, depends on the penetration depth as well as the material's

susceptibility to plastic flow [Evans and Marshall 1981, Toh and McPherson 1986, Shaw et al. 1990 and

1991, Subramanian 1987, and Bifano et al. 1988].

Two models have been proposed to describe the chip formation process shown in Figure 22b.

The Evans and Marshall [1991] model, shown in Figure 11, is based on the formation and propagation

median cracks (vertical to the surface) formed at the boundary between the elastic and plastic zone and

the lateral cracks (parallel to the surface) formed as result of residual stresses during unloading. Crushing

is also an important mechanism of material removal in brittle ceramics [Larchuk et al. 1985]. Crushing

can be described as an unstable fracture process, which involves propagation of cracks at origins

distributed throughout the stressed region. The basic distinction between the crushing model and the

Evans and Marshall model is that in the former a large number fragments are separated from the surface.

Figure 22. Schematic representation of material removal mechanisms: (a) ductile chip formation, and

(b) brittle fracture.

(a) (b)

51
A major consequence of material removal by abrasive action is the generation of damage

associated with plastic deformation, fracture, and heat generation. The type and the extent of machining

damage strongly depends on the type of removal process, machining parameters, as well as the properties

of the workpiece material. Before we describe and classify different types of machining damage, it is instructive to review the types of failure causing defects in ceramics.

Ceramic materials, in general, are susceptible to brittle fracture and, therefore, are sensitive to

the presence of flaws or any inhomogeneities which can act as sources for crack initiation or provide easy

paths for crack propagation. Processing defects such as pores, foreign particles, large grains, or combinations of these are major sources of failure in ceramics [Rice 1979, Rice and Mecholsky 1979,

and Rice et al. 1979]. Service and environmentally induced defects (for example, corrosion pits) are also

important causes of failure. A third source of defects that can severely affect the strength of ceramics

is damage produced by handling (for example, scratches). The processing related defects can be

minimized by controlling the process parameters during powder processing, densification, and sintering.

The service and environmentally induced defects can be minimized through proper design and selection

of materials for each specific application. As most of the aforementioned defects are carefully eliminated,

the machining damage becomes an important strength limiting factor.

The nature of machining damage and its effect on the strength of ceramics has been extensively

studied in the past [Rice 1979, Rice and Mecholsky 1979, Koepka and Stokes 1979, Cranmer et al. 1977,

and others]. Plastic deformation plays an important role in the removal process, even for brittle

materials. Surfaces finished by abrasive processes always contain residual plastic deformation, the extent

of which depends on the material and the parameters of the machining operation [Kirchner 1984, Primak

1981, and Hooper and Morgan 1989]. In certain ceramics, for example tetragonal zirconia, plastic

deformation results in the development of texture and phase transformation [Whalen et al. 1989, Mehta

et al. 1990, and Krishnamurthy et al. 1991]. Another direct consequence of plastic flow is the

development of residual stresses, which can be compressive or tensile on the surface [Kirchner and

Isaacson 1982 and 1986].

Microcracks generated during machining have a detrimental effect on the strength of ceramic

components. The damaging effect of microcracks is depicted schematically in Figure 23, which shows

the cross section of a machined surface perpendicular to the grinding direction [Marshall et al. 1983].

Each abrasive particle produces a small semicircular microcrack in proportion to the penetration depth

of the abrasive particle. Although an individual microcrack may be too small to be critical, a group of

microcracks may act as a larger crack and initiate failure. In the presence of a tensile stress on the

surface, or more importantly at the crack front, the crack can propagate rapidly.

Irrespective of the type of chip formed, most of the mechanical energy associated with the process

of removal is expended as heat. Sliding at the chip/particle and the workpiece/particle interface results

in frictional heating [Malkin and Ritter 1989, Wetton 1969, and Cook 1990]. Heat is also generated by

the shear deformation in the shear zone. Some calculations suggest that approximately 50 percent of the

total heat generated in grinding of metals is associated with the shear zone [Malkin 1989]. The

temperature during grinding of metals can reach as high as 1200°C to 1400°C [Ueda et al. 1985, and

Ramanath and Shaw 1988]. The contact temperature in grinding of ceramics with low thermal

conductivity can be higher than that for metals, since heat is not efficiently conducted away from the

contact zone [Malkin 1989].

52
surface

damage layer

crack front at

instability

Figure 23. Schematic representation of crack configuration generated by abrasive machining [Marshall

et al. 1983].

A direct consequence of the high contact temperatures is chemical reaction between the workpiece

and the environment or the abrasive particle. In addition to chemical reactions, diffusion and segregation

of various components have been observed [Komanduri and Shaw 1976]. The high rates of heating and

subsequent cooling also can give rise to thermal stresses and cracking. It should be noted that the

heating/cooling effect can modify the residual stresses in the workpiece. Although, the effect of interface

temperature and chemical reactions can play a major role in the grinding operation, this phenomenon has

received only limited attention in grinding of metals [Rowe et al. 1991, and Lavine 1991]; and it has not

been investigated for grinding of ceramics.

6.2 Grinding Rate and Damage

Grinding rate is usually expressed as volume removed per unit time. In practice, the achievable

rate of grinding can be limited by a number of factors: the power capacity of the machine, strength and

durability of the wheel, excessive wheel wear, insufficient rigidity to maintain precise dimensions, or the

ability of the workpiece to support the applied forces. Most often, however, it is the need to obtain a

specified surface finish and surface integrity of the part that limits the grinding rate. Here, surface

integrity refers to any form of grinding induced performance limiting damage sustained by the workpiece.

As discussed previously, the damage may take the form of cracks, residual stresses, and transformed or chemically reacted layers.

In the routine machining of advanced structural ceramics, at least as it is practiced by the average

machine shop, the approach is to employ removal rates that are well below those which are thought to cause damage. As a typical example in ordinary reciprocating surface grinding, down feed is limited to no more than 25 jum, utilizing a table speed of 0.25 m/s and a wheel surface speed of 20 m/s, resulting

in a removal rate of approximately 0.6 mm2/s per mm of wheel width. This approach is clearly arbitrary

and sacrifices the possibility of grinding at higher rates with a concomitant increase in productivity, while

still maintaining an acceptable level of damage.

53
In principle, if a large depth of material must be removed by grinding, surface finish and damage

may not be as serious a limitation as it might appear. Rapid grinding to remove the bulk of the material

might be followed by a final finishing stage to remove earlier damage, leaving only damage associated

with the final stage. This approach is, of course, commonly employed in machining metals.

Unfortunately, for advanced ceramics, there is no convenient and directly applicable means to determine

how much material should be removed to eliminate the damage from course grinding. The problem is,

of course, exacerbated by the fact that only one large subsurface crack may be the limiting flaw.

"Large," in the case of low fracture toughness advanced ceramics, may mean only a few micrometers

in length, which is small in terms of detection when many square centimeters of component surface area must be examined. Detection of such flaws exceeds the capability of all but the most tedious and

expensive NDE methods that are currently available.

This leaves proof testing or the preparation of separate test specimens for destructive evaluation

of grinding damage. The latter method, although less satisfactory than evaluation of the actual

component, has been used in several investigations to study the effect of machining variables [Allor and

Baker 1983, Thomas et al. 1987a and 1987b, and Matsuo et al. 1987]. Recently, this approach has

achieved a high level of significance with the issuance of ASTM CI 161-91 Standard Test Method for

Flexural Strength of Advanced Ceramics at Ambient Temperature. In this method the flexural strength

(also referred to as the modulus of rupture, or MOR) of test bars of prescribed dimensions is determined

by three-point or four-point bending tests. The flexural strength and its variability are determined by

7001 I I I I 1 0.04 0.08 0.12 0.16 0.18

DOWNFEED RATE (MM)

Figure 24. Effect of down feed rate on characteristic strength of machined silicon nitride [Thomas et al. 1987a].

54
the inherent properties of the particular lot of material and by any damage which may have been

introduced during machining, finishing, and other treatments. By following a carefully prescribed

preparation procedure set out in the ASTM Standard, the inherent strength of the material can be

determined with good statistical significance without the influence of damage due to other sources. Thus,

the test can then be used to investigate the influence on strength of different grinding and surface

preparation procedures.

Several examples can be cited in which the flexural strength testing has been used to determine

the effects of grinding variables on damage. Thomas et al. [1987a and 1987b] investigated a number of

parameters in grinding hot-pressed silicon nitride and sintered silicon carbide. In Figure 24 the effect

on strength of down feed (depth of cut) is shown for silicon nitride. A significant decrease in strength

is observed for increasing down feeds, confirming the practice of using small cutting depths. Figures 25

and 26 show the influence of table speed (feed rate) and grit size, respectively, on strength of silicon

nitride. At higher table speeds, a statistically significant improvement in strength is obtained. In the case of grit size, as shown in Figure 26, a maximum in strength was obtained with 320 grit. In both cases,

the results are counter intuitive in that grinding at greater removal rates (increased table speeds) might

have been expected to result in increased damage and lower strength; and similarly, the trend towards

increased strength with reduced grit size on going from 150 to 320 grit would have been expected to continue for 600 grit. The latter effect occurred despite the fact that a smoother surface was obtained

with 600 grit, implying that the depth of damage might be smaller.

In other experiments Thomas et al. [1987b] investigated the effect of wheel surface speed and

grinding direction relative to the axis of the test bars. These results are shown in Figures 27 and 28,

respectively. It is seen that increasing the wheel surface speed resulted in higher flexure strength. The

115 I I 1 1 1 J

10 20 30 40 50 60

Table Speed (Feet/Min.)

Figure 25. Variation in characteristic strength of silicon nitride with table speed [Thomas et al. 1987b]

55
140

200 400 600

Grit Size

800

Figure 26. Variation in characteristic strength of silicon nitride with grit size [Thomas et al. 1987b].

140

2000 4000 6000 8000

Surface Speed (SFPM)

Figure 27. Variation in characteristic strength of silicon nitride with wheel surface speed [Thomas et al. 1987b].

56
140

60 I 1 ! L_

0* 45* 90*

Orientation To Tensile Axis

Figure 28. Variation in characteristic strength of silicon nitride with angle between grinding direction

and tensile axis [Thomas et al. 1987b].

surface roughness was also less with increased wheel surface speed suggesting that material removal was

accomplished with a smaller grit penetration depth, which may explain the reduction in damage and

increase in strength.

The angle between the grinding direction and the tensile axis of the flexure specimens is shown

in Figure 28 to have a strong influence on strength. The measured strength was highest when grinding

was carried out parallel to the tensile axis and lowest when grinding was in the perpendicular direction.

Examination of the fracture surfaces indicated that the exposed flaws were larger in the bars ground in

the perpendicular direction. This is apparently associated with the fact that the median cracks, which lie

parallel to the grinding grooves, are larger than the radial cracks, which are perpendicular to the grooves.

An additional source of decreased strength is that the grinding grooves may act as notches when they are perpendicular to the tensile axis.

The effect of grinding variables on the strength of hot-pressed silicon nitride was also studied by

Allor and Baker [1983]. They obtained the same grit size effect observed by Thomas et al. that is, the

strength was higher at 320 grit than at either larger or smaller grit sizes. In addition, little effect was

observed when two different grinding wheel grit concentrations were employed. In a comparison of resin

bond versus plated wheels, they observed a slightly larger strength loss with plated wheels. A similar

result was obtained with resin bond versus metal bond wheels, although they had found a significant loss

of strength in earlier experiments with the same metal bond wheels. They attributed these disparate

results to differences in the skill of the two operators who had ground the specimens. In contrast to the

observations of Thomas et al., Allor and Baker found that down feed rate could be increased from 0.0254

to 0.254 mm/pass with little change in strength, although increasing the down feed rate further to 0.762

mm/pass did result in a small decrease in strength.

57
As a final example the results of Daniels et al. [1989] in grinding sintered silicon carbide are of

interest. Daniels et al. carried out experiments with three different types of diamond grit. One grit type

did appear to perform better than the others, but in no case was there a consistent loss in strength

associated with increasing the down feed from 0.025 to 0.203 mm. The results obtained with one type

of grit are shown in Figure 29. In this case there was in fact an improvement in strength compared with

a carefully prepared control specimen.

The above examples show that changing the grinding conditions does not always have the same

effect. In some cases this may be attributed to the skill of the operator or details of the grinding

procedures. Often, however, it appears that the results are specific to the material being investigated.

Differences in material microstructure, inherent flaw populations, and toughness could account for the

apparent inconsistencies.

MEAN MOR (KPSI) vs. DOWNFEED (INCHES)

CO

0-

CL

Z5

L~

u_o

CO

z>

I QO

STANDARD DEVIATION

iBi wmmm

0.0O3 0.005

5000 SFPM

0.003 0 005

2500 SFPM

0.008

DOWNFEED (INCHES)

Figure 29. Effect of down feed on mean modulus of rupture (MOR) at two different wheel speeds: 2500

and 5000 surface feet per minute (SFPM) [Daniels et al. 1989].

6.3 Methods of Damage Characterization

Different NDE methods have been developed and are being evaluated for flaw detection in

advanced ceramics. However, most of these methods are only suitable for the detection of large flaws.

58
To detect machining damage, the NDE method must be sufficiently sensitive to resolve small flaws,

mostly microcracks, very close to the surface. Also, the entire machined surface is subject to machining

damage, which requires a method capable of examining what is often a relatively large area.

Friedman et al. [1986] evaluated several NDE methods to determine which were most suitable

for detection of machining damage in silicon carbide. The methods evaluated included optical

microscopy, fluorescent dye penetrant inspection, high-frequency surface wave acoustic microscopy,

thermal wave imaging, krypton exposure technique, bubble test, and microfocus x-ray. These methods

were used to analyze a ground surface and a knoop indentation, simulating a larger surface flaw. Only

optical microscopy, surface acoustic wave, and thermal wave techniques detected the Knoop indent.

Also, surface acoustic wave and thermal wave techniques seemed promising for the detection of the

grinding damage, although the results were not conclusive. A similar conclusion was reached in an

international round robin evaluation of machining damage reported by Tonshoff et al. [1989]. They also

concluded that conventional inspection and measurement methods lack in resolution, and indicated that

several damage evaluation techniques should be used for the analysis of different types of damage

introduced by machining.

Visual inspection utilizing liquid penetrants such as fluorescent liquids and white and red dyes, is simple and inexpensive. Although, this method can not detect subsurface cracks and small surface flaws,

it can be used for larger surface cracks. In fact, this is the method of choice in most machine shops.

The principle governing the operation of the surface acoustic wave method is illustrated in Figure 30,

and a schematic of a complete detection system is shown in Figure 31 [Fahr et al. 1984]. The feasibility

of using this method has been evaluated for machining damage, a simulated damage introduced by a

Figure 30. Generation of leaky surface acoustic waves (SAW) and their interaction with a surface crack

[Fahr et al. 1984].

59
focused

transducer J*

Specimen | A

Pulse

generator —»

Amp

Protection

circuit

Analog

signal

display

digital

signal

display

A to D converter

Microcomputer

waveform

storage

I

^User's \ i Graphics,

terminal \

Plotter Magneti

tape 1

Figure 31. A schematic of a surface acoustic wave detection system and the computer analysis facilities

[Fahr et al. 1984].

scratch [Clarke et al. 1986, and Khuri-Yakub et al. 1984] and propagation of cracks from scratches

[Marshall et al. 1983]. Variants of the acoustic wave microscopy technique which are promising for the

assessment of machining damage are shown in Figure 32.

Photothermal microscopy and particularly thermal wave imaging are also promising techniques

for the assessment of machining damage. The basic principle of operation of the latter method is shown

schematically in Figure 33. This method essentially measures the thermal diffusivity of the surface and

a thin subsurface layer. Therefore, machining damage that has an effect on thermal diffusivity can be

detected. An example of results obtained on silicon nitride abraded with different sizes of diamond grit are shown in Figure 34 [White 1992]. The data clearly show that there is a correlation between thermal

diffusivity and the grit size used to abrade the surface. The observed effect can be attributed to the larger

amount of damage produced by the larger particles.

In addition to detection of microcracks, it is important to assess the nature and magnitude of

residual stresses introduced by machining. Perhaps the most suitable technique is x-ray diffraction, which

has been evaluated by several investigators [Johnson-Walls et al. 1984 and 1986, Pfeiffer 1990, Pfeiffer

et al. 1988, and Eigenmann et al. 1989]. Generally, residual stress is compressive on and near to the

60
surface. For example, Johnson-Walls et al. [1989] measured a compressive residual stress of 350 MPa

in a silicon nitride surface. The measured thickness of the layer with compressive residual stress was 10

fim. It has been reported that the magnitude of the compressive residual stress increases by increasing

the depth of cut in diamond grinding [Praddel et al. 1989]; this is perhaps due to a larger amount of

plastic deformation for large depths of cut. Although, a compressive surface stress would be expected

to increase the strength of a material, experimental results show the opposite. This can be explained by

considering the fact that compressive surface residual stresses are always compensated by residual tensile

stresses below the surface. If subsurface cracks are located in the tensile region, then the surface

compressive stress is ineffective in increasing the strength; and, in fact, the tensile residual stresses cause

a reduction in strength.

Receiver CW- Loser

)us CW- Laser

transmitter

\^~s^/ microphone^

Chopper

IR- detector

USreceiver

SLAM SAM PAM

Figure 32. Principles of scanning laser acoustic microscope (SLAM), scanning acoustic microscope

(SAM) and photo-acoustic microscope (PAM) [Goebbles 1987].

In a recent review of nondestructive measurement techniques, Brinksmeier [1989] concluded that

x-ray diffraction, ultrasonic methods, photothermal microscopy, Raman spectroscopy, and instrumented

nano-indentation are promising techniques for the evaluation of machining damage. Raman spectroscopy,

which utilizes penetration of light in superficial surface layers, is an important tool for studying the

structure, chemical composition and stresses in crystalline and amorphous materials. The instrumented

nano-indentation, however, is a quasi-nondestructive technique for evaluating mechanical properties such

as hardness and elastic modulus of superficial surface layers.

Other NDE techniques that could be used for machining damage assessment include optical and

holographic interferometry [Roszhart 1973, and Goebbles 1987]. These techniques, however, primarily

measure the surface profile, and surface roughness.

61
TW: mirage effect

1

Periodic Heating

Probe Beam

heated region

Specimen

Figure 33. Schematic representation of thermal wave (TW) mirage effect [White 1992].

0.27

S3

0.22 I

i i i i i i i i i

i i i i i i i i i ii

i i i i i i i i i

i

i i i i i i i i i 0 10 20 30 40

Grit Size (micrometers)

Figure 34. Effect of grit size on thermal diffusivity [White 1992].

62
6.4 Suggested Technical Approaches

It is recommended that the types of machining damage in different materials under various

machining techniques be identified and classified, and that the effect of machining parameters on the

extent of machining damage be evaluated. Furthermore, the most detrimental types of damage for

different applications must be identified, and the most reliable NDE techniques for detecting such damage

should be identified.

In order to increase the material removal rate and at the same time reduce the extent of machining

damage, one must recognize the complexity of the grinding operation and analyze the different parts of

the process. At the microscale, the interaction between the abrasive particles and the workpiece controls

the rate of removal and the type of damage produced. The important parameters controlling the processes

at the interface include contact temperature, interfacial forces, and the environment surrounding the

cutting zone. The interfacial actions, however, are influenced by the grinding parameters, and by the

machine tool characteristics. Therefore, a possible approach for increasing the machining rate in a total

grinding system, is optimization of the process through automation and control. This objective requires

research and development efforts for sensors to assess the machining damage and surface quality during

the operation. For certain applications sensitive to small amounts of machining damage, one may

consider post-machining treatments to remove the machining damage and proof testing to increase

performance reliability.

63

7. RECOMMENDED RESEARCH FOR CERAMIC MACHINING

In order to take advantage of the unique properties of advanced ceramics and facilitate their

introduction into the market place, a coordinated program is needed. The overall objectives of this

program should be to develop an advanced technology for ceramic machining which will remove the

barriers that machining presents to the widespread introduction of advanced structural ceramics. The

recommended research program outlined in this section is based on our discussions with industry and the

analysis of published literature. It is recommended that these projects be carried out jointly with industry

to facilitate technology transfer, and that the ceramic machining research funded by the government be

coordinated to minimize the possibility of duplication of effort.

7.1 Summary of Needs

The results of the assessment with regards to the needs in the machining and production of parts

made from advanced ceramics are summarized as follows:

Reduction in Cost: Machining is a major contributor to the overall high cost of ceramic

components. Although, significant costs are also associated with raw materials, processing, and

quality control, the primary impediment to the introduction of many ceramic parts into the market

place is the high cost of machining. Therefore, research efforts should be focused on developing

methods which reduce machining related costs as a means of promoting the manufacture of costeffective ceramic components.

Optimization Data: With the exception of a few publications, there is little machining data

available for advanced ceramics. There is a critical need for data to be used for the purpose of

cost estimating, component design optimization, and selection of machining parameters. In the

absence of such information, the operator takes the most conservative route, i.e., slow removal

rates, which translates to a high cost of machining.

Rapid Machining Methods : Advanced ceramic materials are difficult to machine with most

conventional methods, with the exception of abrasive machining utilizing diamond. Abrasive

machining of advanced ceramics, however, is slow compared to metals. Innovative techniques

are needed to increase the rate of material removal. But it is extremely important to recognize

that an increase in machining rate may lead to an unacceptable level of surface and subsurface

damage in the finished part. Therefore, the development of rapid machining methods must be

accompanied by damage assessment and control.

Damage Assessment: Performance and reliability of ceramic materials are strongly influenced

by damage introduced during machining. Surface finish and machining damage are especially

sensitive to small changes in machining conditions. There is a need for reliable and fast, in situ

and post-machining damage assessment techniques. Development of sensors for real-time

measurement of surface finish and damage assessment has the potential to increase manufacturing

productivity significantly.

Automation : Present ceramic machining practice is oriented towards labor-intensive small-scale

production and heavy reliance on operator skill. If advanced ceramics are to be extensively used,

65
automated production techniques are needed to make millions of components. This requires

additional research to develop new machine tools, special part-handling techniques, and on-line

inspection to control machining damage, surface finish and part dimensions.

7.2 Recommended Research Projects

7.2.1 Optimization of the Grinding Process

Our discussion with industry indicated that one of their most pressing needs was for information

on how to optimize the grinding process to achieve maximum removal rate, minimum residual damage,

and lowest cost. As already stated, grinding involves a large number of interdependent parameters, such

as, feed, depth of cut, wheel speed, table speed, machine tool characteristics, grinding fluid chemistry,

and wheel type. The machining rate and the amount of residual machining damage are very sensitive to the selection of these parameters for each specific material. The main focus of this program activity

should be to develop and disseminate such data.

A group of industrial and government participants, having a sound background of experience in

grinding advanced ceramics, should be assembled for this project. Different types of ceramics, including

silicon nitride, silicon carbide, and composites should be used. All samples should be completely

characterized before machining as to structure, residual stress, porosity, impurities, etc. These test samples should be machined to various degrees of severity by the industrial partners. The purpose here

is not only to evaluate the effect of different variables on the quality of finished surface but also to determine the variability of current practice. Surface finish and grinding damage should be assessed using

the most appropriate techniques. The true test of various machining techniques is to determine how that

process affects the performance of the material. For ceramics, strength, wear, fatigue, and microhardness

are particularly important for most applications. The test samples which have been fully characterized

and machined to different degrees of severity should be measured for these properties. With these

measurements, a correlation can be established between machining severity and the selected properties.

The machining data and the information regarding strength and performance may be assembled and

widely distributed in computerized database format.

7.2.2 Chemically Assisted Machining

As already discussed in the previous chapters, many machining methods have been suggested and

are being evaluated. However, progress has been slow due to various technical problems. We must

recognize that the simplicity of the machining method and the effectiveness of technology transfer are the

primary factors which influence the implementation of research results. One approach which satisfies

these requirements is to find innovative means for improving present machining methods. One area which has not received sufficient attention concerns the application of chemical assistance or enhancement. Three methods merit attention with respect to the application of chemical assistance:

grinding, ultrasonic machining, and liquid abrasive jet cutting. Since the mechanics of material removal

are similar in each of these techniques, the chemical assisted technology could first be developed for

grinding and later applied to the other processes.

In grinding, it is possible to have significant chemical interactions between the local environment

in a grinding system and any or all of three elements: the workpiece, the grinding wheel matrix, and the

diamond abrasive. The simplest, and perhaps most economical method for the delivery of chemical

compounds to the cutting zone is through the grinding fluid. Therefore, studies should investigate the

66
effect on the grinding process of chemical compounds added to the grinding fluid and should identify the

significant chemo-mechanical interactions that will increase the rate of material removal while minimizing

the grinding damage. The outcome of this project, transferred to industry, would be a set of

recommendations for grinding fluid additives.

7.2.3 Grinding Wheels for Machining of Ceramics

Research in this area should be focused on the development of grinding wheels and dressing

techniques for application to advanced ceramics. In reviewing the critical role that the grinding wheel

plays in the grinding process, it was pointed out that wear of the diamond grit was of overriding

importance. Grit wear translates directly into higher grinding energies, increased workpiece damage,

greater machine tool deflection, higher temperatures and associated degradation of the grinding fluid, and

a reduction in the maximum achievable material removal rate. Wear of the grit is determined by the

properties of the diamond used, the workpiece properties, grinding fluid chemistry, and the grinding

parameters. Systematic investigation of the relationships among these variables should be carried out.

Quantitative information on the extent to which grit wear influences workpiece damage is also

of critical importance. Such information could be used to determine the wheel dressing interval, or dressing rate if continuous dressing was employed. An associated effort would be the development of

sensors and instrumentation to monitor wheel wear.

Although there appear to be significant opportunities to improve wheel wear life through grinding

fluid chemistry and other means, it is unlikely that the problem of grit wear can be eliminated. An

important requirement is that wheel wear be just sufficient to eliminate dull grit and expose fresh, sharp

grit. Given the often conflicting requirements on bond properties, usually it is not possible to incorporate

the necessary degree of controlled bond wear to accommodate grit wear. One solution to this problem

is through in-process programmed wheel dressing; that is, wheel dressing would be accomplished during

grinding at a rate or in intervals required to maintain sharp grit and/or a suitable level of grinding

efficiency. Implementation would require the development of dressable-bond materials, cutting fluids, sensors to detect wheel wear, and adaptive control to accommodate dimensional changes associated with

dressing.

7.2.4 Direct Damage Assessment Techniques

A review of the current state of the technology shows that there are no standard methods — nondestructive or destructive — for the direct determination of machining damage. Because of the

importance of damage control in the machining of advanced ceramics, such investigations should receive

a high priority. A direct method for damage evaluation is suggested in this section; other NDE

techniques are discussed in the following section.

The most widely used indirect method for the evaluation of machining damage is the four-point

bend test. This test is used to measure the flexural strength of ceramics and can be used to determine

the effect of machining damage on strength. However, the bend test typically requires a minimum of 30

specimens, all machined under the same conditions to a close tolerance. Therefore, the bend test can be

quite expensive for the determination of the machining damage. While the bend test may be suitable for

determining flexural strength, it is not well suited for the evaluations of surface damage that might affect

other types of performance, for example sliding wear or contact fatigue. A possible technique to employ

67
is relatively new, and has been used to determine the mechanical properties (modulus of elasticity,

hardness, and fracture toughness) of thin surface layers. The depth of indentation can be controlled to within a few tens of nanometers. This technique can be also used to measure the depth of the damage

layer. It should be emphasized that the apparatus is already available at several laboratories, but much

research is needed to interpret the resulting data on machined surfaces. This project has the potential for

establishing a standard test procedure, based on nano-indentation for machining damage evaluation.

7.2.5 NDE Techniques for Damage Evaluation

In the past twenty years substantial effort has been devoted to the development and evaluation of

NDE techniques for advanced ceramics. The basic requirement in the quality control of advanced

ceramics is surface flaw detection in the 10 /*m range or less. This is an order of magnitude below that

commonly necessary for metals. Accordingly, a variety of new approaches have been developed for

ceramics with various degrees of success. Application of these techniques to the evaluation of machining

damage has been limited. For machining damage, it will be desirable to have techniques which give a general measure of damage as well as identifying the largest flaw. Our review of the technical literature,

has indicated that the most appropriate NDE techniques are thermal wave measurement, surface acoustic

wave, ultrasonics, and x-ray techniques.

The capability of these techniques to assess machining damage should be evaluated by analyzing test samples which have been machined with different severities from polishing to rough grinding. The

results should be used to establish the degree to which each technique measures the change of severity.

It is also important to use performance tests and known destructive techniques to evaluate the

effectiveness of the NDE techniques. In this investigation, the types of damage detrimental to

performance should identified. The results of this research can be used to refine the selected NDE

techniques and to develop standard procedures for the utilization of these methods for surface damage

assessment.

7.2.6 Sensors for Real-Time Surface Finish and Damage Assessment

The ability to control the grinding process in real time and obtain the desired surface finish could

lead to a significant increase in productivity. Two approaches should be considered. The first is to develop sensors to directly measure surface finish and damage. At this time optical and ultrasonic

methods appear to be the most promising. Projects should be undertaken to develop these methods and

use them to control grinding parameters. The second approach involves indirect measurements to control

the grinding parameters. These include grinding forces, power consumption, and acoustic emission. The

relationship between these quantities and surface finish, and damage, together with the effect of grinding

parameters, would have to be established through research.

7.2.7 Standard Reference Materials

It is often necessary to compare the quality of a finished part with a reference standard in order

to evaluate the success of the machining technique with respect to the residual damage and surface

roughness. It is recommended that fully characterized standard ceramic specimens be developed with a minimum amount of existing damage for evaluation of the quality of machining techniques. The samples

must be provided from a single lot of material, in a geometry suitable for the preparation of four-point

bend specimens and, if necessary, for other tests. Characterization of the material should include fracture

toughness, surface roughness, surface residual stress, and other properties. Industrial laboratories could

68
toughness, surface roughness, surface residual stress, and other properties. Industrial laboratories could

purchase these standard reference materials (SRM) and machine them according to their practice. The

quality of the finished surface could then be compared with similar data for the SRM to determine

whether machining had severely degraded the integrity of the finished surface.

7.2.8 Post-Machining Treatments for Damage Control

Removal of material by any mechanical method will result in a certain amount of damage in the

form of lattice defects, microcracks, and residual stress. A possible approach to damage control other

than optimization of the machining processes is to develop effective post-machining treatments that

mitigate the damage. The following methods should be considered: high temperature oxidation, high

temperature annealing, chemical etching to remove damage, ion exchange, tempering, and coatings. Each

of these areas must be explored in detail to determine the effectiveness and the economics of the process.

7.2.9 Innovative High-Strength Machinable Ceramics

Past research has established that additions and impurities incorporated within materials can have

significant effects on mechanical properties and machinability. An approach that has been successfully

followed in metals and glass, that of intentional additions to enable satisfactory machining, may have

some promise in the case of structural ceramics. An example of this approach is the development of

electrically conductive ceramic composites from which parts can be fabricated by electrical discharge

machining. These composites are generally made by adding electrically conducting particles to a nonconductive ceramic such as alumina. The additions that have been successfully implemented include

TiC, TiB2 , ZrB2 , and TiN. To overcome the decrease in strength that may result from the addition of

the electrically conductive particles, often SiC whiskers are incorporated into the composite [Schuldies

and Branch 1991]. Addition of second phase particles to a ceramic composite does not always decrease

the strength and fracture toughness. It has been shown by Benison et al. [1991] that addition of

aluminum-titanate to alumina increases the material's resistance to fracture as a result of crack bridging,

crack deflection, and compressive residual stresses set up during processing. These results suggest that

if a sufficient understanding is developed on the interactions between second phase particles and the

ceramic matrix, it would be possible to design machinable ceramics for structural applications. This

project requires detailed investigation of processing of ceramic composites to optimize the processing

parameters for best machinability and strength. In addition to electrical discharge machining, it may be

possible to develop ceramic composites with greater machinability by the abrasive processes such as grinding.

7.2.10 Automated Systems for Large-Volume Production

Significant advances have been made in automation and the development of intelligent systems

for the production grinding of metal components. Similar systems for the grinding of ceramic

components involve additional requirements and challenges. The first requirement for such a system is reliable, fast-acting sensing devices for surface finish assessment and damage detection. The information

from the sensors could be fed into a computer for comparison with a set of preprogrammed information.

This could be in the form of a knowledge-based system or an analytical model, which would then be used

to search for, or calculate, the best parameters for an optimized machining condition. This information

would then be transmitted to the control circuitry of the machine tool. Implementation would require the

development of machinability databases, knowledge-based systems, and analytical models for grinding

optimization, in addition to sensor development. Other important factors that must be considered in an

69
automation system are machine tool dynamics, wheel truing and dressing, and parts handling adapted for

ceramic materials. To assess the feasibility of intelligent systems for the production of ceramic parts, a model system should be designed for the production of a specific component, for example a silicon nitride

roller-cam follower.

7.3 Program Coordination

The program objectives described are far beyond the capabilities of a single organization. It is estimated that such a program would require several million dollars over a 5-year period. Thus, a

coordinated program is necessary which utilizes the skills, experience, and resources from several

organizations.

Several government agencies have recently initiated research programs related to ceramic

machining. The Department of Energy, Office of Transportation Materials, has started a program on

cost-effective ceramics, which includes ceramic machining as one of the program elements. The Defense

Advanced Research Projects Agency, Ceramic Materials Program, is now funding several projects to

accelerate the introduction of advanced ceramics into military and civilian applications. Some of these

projects are concerned with machining and fabrication techniques for ceramic components. The Air

Force Manufacturing Technology Program is formulating a 5-year program primarily focused on machine

tools and their improvements. One aspect of this program deals with machining of advanced materials.

It is important to recognize the need for coordination among these agencies to eliminate the potential for

duplication of effort and to advance the technology of ceramic machining through joint efforts.

7.4 Technology Transfer

The information generated in the course of research to resolve the identified needs must be

transferred to those who can implement the technology. It should be emphasized that publication of

research results in archival journals and reports is slow, and often does not reach the intended audience.

Therefore, other direct forms of communication are needed for rapid technology transfer. In this respect,

it is important not only to conduct the proposed research in close collaboration with industry, but also

to organize frequent lectures and symposia to disseminate the research information generated in this

program.

70
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81
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83

Appendix A

NIST CERAMIC MACHINING WORKSHOP

1. Summary

A workshop meeting was held on September 19, 1990, at NIST to identify those topics in the

field of ceramic machining and finishing which industry felt were critical to the advancement of this field.

Twenty-one participants from industry attended. Presentations by industry described both the nature of

research that was felt to be important in this area as well as the impact of machining on the cost of

ceramic components. A tentative plan of research was developed and industrial participants were

requested to respond to the Ceramics Division with comments. It was agreed that industry and

government should jointly sponsor a program to address the short-term and long-range needs of the

industry to overcome technology barriers.

2. Background

The workshop was organized following discussions with personnel from the ceramic companies

and the United States Advanced Ceramic Association. These discussions indicated that commercialization

of advanced ceramics for transportation systems was adversely impacted by the cost of machining. The

objective of the meeting was to identify critical research priorities in this field and opportunities for

cooperative research between the NIST and the ceramic industry.

Representatives from the structural and electronic ceramics industry as well as users and machine

tool manufacturers were invited to this meeting and provided the opportunity to present their views and

perspectives. Twenty-one individuals from industry participated and are listed in Appendix B. In

addition to industrial participants, the U.S. Department of Energy and the United Stated Advanced

Ceramic Association were also represented. The meeting was also attended by NIST personnel from

three divisions, many of whom presented research results pertinent to ceramic machining.

3. Proceedings

Industrial representatives from Allied Signal/Garrett, W. R. Grace, Co., Carborundum Company,

the Norton Company and the Advanced Manufacturing Science and Technology Co., all reported that

reduction of machining costs is an important factor in the widespread utilization of advanced structural

ceramics. Specifically, it was felt that although desired tolerances and finishes were achievable by

machining, the opportunity was present to (1) radically reduce the cost of machining and (2) to

significantly improve the quality of surface finish, hence, reliability.

Several industrial participants identified machining and finishing as a primary cause of the high

cost of ceramic products. Costs ranged from 10 percent to 15 percent of the total cost of a product where

a single operation was required for over 100,000 parts to as high as 80 percent where multiple operations

were required for fewer than 25 parts. Other companies reported that for specific components the cost

of machining to the preformed stage could account for anywhere from 52 percent to 72 percent, whereas

final machining could run as high as 80 percent to 90 percent of the final product cost. A summary of

the estimates given by industrial participants is provided in Table A-l.

85
TABLE A-l. COST OF CERAMIC MACHINING/FINISHING

Machining cost as % of sales

Electronics 17%

Mechanical Seals 17%

Armour 18%

Bearings 55% to preform

80-90% to finish

Prototype Components 62%

Machining cost as a function of number of parts

Less than 25,000 20-80% of direct material cost *

25-100,000 40% of direct materials cost*

More than 100,000 10-15% of direct material cost**

* Multiple Operations

** Single Operation

Machining cost by type of operation

Centerless grinding 3-10%

Rotary Table 15-50%

Free Abrasive 15-30%

86
The following list of research needs were identified by the various industrial speakers present:

• Reduction in cost of materials, processing and machining required to make ceramic components

economically competitive with metal components was agreed to be critical to the future of ceramics

technology.

• Novel processes of machining which would allow rapid material removal as well as minimal

surface damage to increase part reliability was identified as a desirable goal. Chemically assisted

machining for mass production was identified as a high payoff research topic.

• Production volume to price interaction for a given ceramic material requires improved

definition particularly in the context of a lower cost approach to manufacturing.

• Mechanisms of machining of ceramic parts require better understanding both on fundamental

and applied levels. This knowledge could be readily utilized by designers of machine tools and grinding

wheels as well as ceramic component finishers.

• Effect of machining parameters on performance is not well understood. Experiments were cited

which showed that machining rates could be significantly increased without a penalty on stress rupture

properties. However, the limits of increase possible are unknown and specific relationships between

performance and machining rate are unclear.

• Creative automation technology was cited as important and potentially of great use in the field

of machining, particularly as it relates to controlling surface properties and to reducing costs.

• Identification and quantification of machine tool characteristics which control the quality of the

finished product were cited. Also cited was the necessity for working directly with manufacturers to

develop improved grinding machines. Available machines designed for metal working have an

insufficient inherent stiffness to allow significant ceramic material removal without adverse deflection of

the machine tool elements or the material itself.

• Understanding the nature of grinding wheels and grinding wheel design were identified as areas which could provide significant improvements in the machining process. Well-understood and defined

wheel specifications were felt to be limited.

• Research on the nature of grinding fluid behavior and properties was identified as a potentially

significant research area. Currently, specific grinding fluids for ceramic machining are not available and

it is not clear whether fluids available for metal machining are entirely appropriate for ceramic

applications.

• Masurement of surface roughness, flatness, and other dimensional features was felt to require

significant attention particularly with regard to the cross referencing of standards between the United

States and Europe.

• Grinding machine to surface interactions with regard to the nature of the finish, residual stress

and surface and subsurface damage were identified as important. Most of the attendees felt that the

ability to measure damage was important to determining the proper machining parameters necessary to manufacture reliable parts.

87
• Accuracy of machining cost projections requires improvement. Two companies specifically

cited this as an area in which they were only moderately able to make accurate estimates to allow bidding

on contracts. From this discussion came the suggestion that a machinability center which could determine

basic data for industry, which relates machining parameters, machine tool behavior, and surface finish

to cost would be a valuable investment.

Another institutional issue which was raised was the need for an interdisciplinary approach to the

grinding of ceramic materials. This would necessarily involve the participation of tool makers, wheel

makers, instrumentation experts and ceramic manufacturers. In spite of the fact that the ceramic

manufacturers procure tools and other equipment from suppliers, they felt the size of their market was

insufficient to encourage the tool manufacturers to develop tools specifically for ceramic machining.

4. Research-in-progress

Several NIST staff members described research underway which addressed the needs of the

ceramic machining and finishing community. Said Jahanmir, of the Ceramics Division, highlighted

research on the effect of surface stress on crack formation and subsequent removal of materials during

grinding. He also presented data which identified the variation in material removal rates which can be

attributed to chemical activity of lubricating or cutting fluids. Differences of about an order of magnitude

were shown for a variety of frictional loading levels.

Also noted was the development of techniques of measuring surface and subsurface structures.

A presentation by Grady White, of the Ceramics Division, highlighted recent research in the use of

thermal wave analysis to determine the variation of surface finish and subsurface machining damage. The

work was felt to have a significant application in determining the predicted performance and reliability

of finished components.

A report by Gerry Blessing, Automated Production and Technology Division, highlighted research

on the use of ultrasonic interrogation to determine surface finish. It is interesting to note that this

research addressed the use of cutting fluids as an ultrasonic transmission medium from the transducer and

sensor head to the workpiece itself. Significant correlations between the results obtained by ultrasonic

interrogation and subsequent profilometry were shown. It was felt that this technique would have a

significant impact for on-line machine tool control.

A fourth talk by NIST staff was presented by Chris Evans who briefly described diamond singlepoint turning and ductile-regime grinding research conducted in the Precision Engineering Division. This

research identified the role of diamond machining of high surface finish ceramic mirror materials.

Following the presentations and a tour of the Automated Manufacturing Research Facility,

potential future interactions in the machining area were discussed. A hypothetical project in this area which could be cooperatively addressed by both government and industrial researchers was presented by

NIST. This plan addressed rapid machining techniques as well as sensor development. Intellectual

property rights, in terms of cooperative R&D agreements were detailed by S. M. Hsu, Ceramics

Division. The participants were requested to review the proposed work and to discuss it with their

management prior to the USACA meeting of October 2, 1990.

88
Appendix B

ATTENDANCE AT THE NIST CERAMIC MACHINING WORKSHOP

K. Anderson

Engineering Manager

Diamonite Division of W. R. Grace

454 West McConkey Street

Sheve, OH 44676

G. V. Blessing

Automated Production and Technology

Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

S. J. Dapkunas

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

T. Estler

Precision Engineering Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

C. Evans

Precision Engineering Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

R. Fouts

Ceramic Process and Development

AC Rochester (GM)

Department 32-34-Plant AC West

1300 North Dort Highway

Flint, MI 48556

S. Freiman

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

H. Friedericy

Director

Research and Technology

Allied Signal Aerospace

25th Floor

1000 Wilson Boulevard

Arlington, VA 22209

R. Gates

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

M. Hancock

Director, Electronic and Structural

Ceramics Committees

United States Advanced Ceramics

Association

1440 New York Avenue, NW

Washington, DC 20005

S. M. Hsu

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

L. Ives

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

89
S. Jahanmir

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

M. Kreppel

Senior Mechanical Engineer

Carborundum

Niagara Falls, NY 14302

P. Lacey

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

R. Licht

Technical Manager

Government Programs

Norton

Goddard Road

Northboro, MA 01532-1545

R. A. Long

Engineering Manager

GTE Products Corporation

Towanda, PA 18848

S. Luszcz

Manager Characterization

Lanxide Corporatin

One Track Industrial Park

Newark, DE 19711

R. MacManus

Research Designer

Brown and Sharpe Manufacturing Co.

French Town Road

N. Kingstown, RI 60852

B. J. McEntire

Operations Manager

Norton/TRW Ceramics

Northboro, MA 01532-1545

K. Metcalfe

Engineering Specialist

Dupont

101 Beech Street

Wilmington, DE 19880

D. Owens

Senior Mechanical Engineer

Carborundum

Niagara Falls, NY 14302

A. Pasto

PMTC, GTE Labs

40 Sylvan Road

Waltham, MA 02254

M. Peterson

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

J. W. Picone

Norton

1 New Bond Street

Box 15008

Worcester, MA 01615

B. R. Powell

Structural Ceramics Metallurgy Dept.

General Motors Research laboratories

30500 Mound Round

Warren, MI 48090-9055

90
R. W. Rice

Director

W. R. Grace and Company

Route 32

Columbia, MD 21044

S. Salmon

Advanced Manufacturing Science

and Technology Co.

P.O. Box 40469

Cincinnati, OH 45240

M. C. Shen

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

J. Sibold

Director, Materials Engineering

Coors Ceramics Company

17750 W. 32nd Avenue

Golden, CO 80401

K. Tadler

Senior Development Enigneer

DuPont Company

101 Beech Street

Wilmington, DE

G. White

Ceramics Division

National Institute of Standards

and Technology

Gaithersburg, MD 20899

W. L. Winterbottom

Manager, Material Engineering

Ford Motor Company

Dearborn, MI 48121

D. Wirth, Jr. Coors Ceramics Company

17750 W. 32nd Avenue

Golden, CO 80401

Appendix C

ORGANIZATIONS AND INDIVIDUALS CONTACTED FOR INFORMATION

Joe Annese

Bomas Machine Specialities

334 Washington Street

Somerville, MA 02143

Ralph Kelly

Cincinnati Milacron

P. O. Box 9013

Cincinnati, OH 45209

"Buddy" Bergoin

Webb Industries

P. O. Box 30700

Raleigh, NC 27622

Ranga Komanduri

Oklahoma State University

Mechanical and Aerospace Engineering Dept.

218 Engineering North

Stillwater, OK 74078

Borges Technical Ceramics

6 Hardwick Street

Belvidere, NJ 07823

Steven Brown

Bullen Ultrasonics

4613 Camden Road

Eaton, OH 45320

Chand/Kare Technical Ceramics

73 Webster Street

Worchester, MA 01603

Robert Kopp

Norton Company

1 New Bond Street

Worcester, MA 01615

Kris V. Krumar

GE Superabrasives

6325 Huntley Road

Worthington, OH 43085

Laserace Technology Corp.

3201 Delany Road

Waukegan, IL 60087

Bruce Cunningham

Accuratus Ceramic Corp.

Brass Castle Road

Washington, NJ 07882

George Ellis

Ellis Ceramatech Inc.

215 Annandale Center

Clinton, NJ 08809

John Indge

Peter Wolters of America

P. O. Box 1585

High Street

Plainville, MA 02762

Lasermation

2703 N. Broad Street

2nd Floor

Philadelphia, PA 19132

Ron Latanison

Metallurgy Dept.

MIT

Cambridge, MA 02139

Ray Middleton

Watertown Arsenal

Watertown, MA 02172

93
Keith Reckling

Diamond Abrasives Corp.

35 West 45th Street

New York, NY 10036

Robert Reeber

Army Research Office

Research Triangle Park, NC 27709

David Rostoker

Norton Company

1 New Bond Street

Worcester, MA 01615

Paul Sampson

Manufacturing Technology

WRDC/MTP

WPAFB, OH 45433

Ron Scattergood

Mechanical Engineering Dept.

NC State University

Raleigh, NC 27695

Milton Shaw

ECG 247

Arizona State University

Tempe, AZ 85287

Charles Shelly

Insaco Inc.

Box 460

Quakertown, PA 18951

K. Subramanian

Norton Company

1 New Bond Street

Worcester, MA 01615

Robert West

Therm Advanced Ceramics

P. O. Box 220

Ithaca, NY 14851

Allen P. Wherry

Grinding Wheel Inst. 712 Lakewood Center

14600 Detroit Avenue

Cleveland, OH 44107

Jim Wollenweber

Laser Applications Inc. Towsen, MD 21157

94
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102
NIST-114A

(REV. 3-90)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

PUBLICATION OR REPORT NUMBER

NIST SP-834

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBLICATION DATE

June 1992

4. TITLE AND SUBTITLE

Ceramic Machining: Assessment of Current Practice and Research Needs

in the United States

5. AUTHOR(S)

S. Jahanmir, L. K. Ives, A. W. Ruff, and M. B. Peterson

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE . NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY (MSEL

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GAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

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Same as item 6.

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

A study was conducted to assess the current state-of-the-art in the machining of advanced

ceramics and to identify research areas which could lead to significant improvements. In conducting the

assessment, an extensive literature search was carried out, visits and discussions were held with industrial

companies interested in ceramic machining, a telephone survey was conducted on ceramic machining

shops, a research-in-progress database was consulted, individuals were invited to visit NIST and discuss

different aspects of ceramic machining, and a workshop was held at NIST in September 1990, to identify

specific industrial needs in ceramic machining.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

advanced ceramics; grinding; machining; manufacturing; polishing; silicon nitride;

technology development

13. AVAILABILITY

UNLIMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

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106

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ELECTRONIC FORM * U.S G.P.0. : 1 992-31 1 -891 :60483



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